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Engineering
Library

THE MOTOR AND THE DYNAMO.

Published by

The Chemical Publishing Co.

East on, Penna.
Publishers of Scientific Books

Engineering Chemistry Portland Cement

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THE MOTOR
AND THE DYNAMO

JAMES LORING ARNOLD, PH.D.

PROFESSOR OF ELECTRICAL ENGINEERING,
NEW YORK UNIVERSITY

EASTON, PA.

THE CHEMICAL PUBLISHING CO.
1913

LONDON, ENGLAND:
WILLIAMS & NORGATE

14 HENRIETTA STREET, COVENT GARDEN, W. C.

?• 7

Library

PREFACE.

This book is the result of many year's experience in present-
ing the essentials of electrical science both to college students
and practical electricians. It embodies the substance of labora-
tory conferences and class room explanations. In every instance
these are based on modern types of machines, to the exclusion
of antiquated models. An unusually large number of illustra-
tions are inserted for the purpose of dispensing with lengthy
descriptions; and thanks are due to various manufacturing com-
panies whose bulletins have furnished numerous half-tones for
these pages.

The author hopes that the book may be found practical
and direct and sufficiently exhaustive by both college men and
electricians.

J. L. ARNOLD.

NEW YORK UNIVERSITY,
November, 1912.

271152

CONTENTS

CHAPTER I

INTRODUCTION

CHAPTER II

MATHEMATICAL PRINCIPLES

Page

(a) Definitions 4

(b) The Induced Current 6

(d) Magnetization Curves 9

(e) The Flow of Current I0

CHAPTER III
THE DYNAMO MACHINE

(a) Frame and Field Cores 17

(b) Field Windings 21

( c) Armature Core 23

(d) Armature Windings 25

(e) Commutator 33

(f) Brushes 35

(g) Brush Holders 35

(h) Bearings 37

CHAPTER IV
OPERATION AND CHARACTERISTICS OF THE D. C. DYNAMO

(a) Preliminary Tests 39

(b) Building-up Curve 46

(c ) Magnetization Curve 47

(d) Armature Reaction 49

(e) External Characteristics 51

(f ) Armature Characteristic 53

(g) The Compound Generator 54

(h) Sparking 56

(i) Operation of D. C. Shunt Generators in Parallel 58

(j) Operation of D. C. Compound Generators in Parallel 61

(k) D. C. Generators in Series 62

(1) D. C. Arc-light Dynamos 64

CONTENTS V

CHAPTER V

THE D. C. MOTOR

Page

(a) Operation and Characteristics • 66

(b) Varieties of Field Excitation 74

(d) Starting-boxes and Controllers 90

(e) Motor Uses 93

( f ) Traction Motors 93

(g) The Motor-Dynamo 97

(h ) Losses of Power in Generators and Motors 98

CHAPTER VI

THE ALTERNATING CURRENT AND ITS MEASUREMENT

(a) The A. C. Wave 105

(b) Mean, Average and Effective Values • , 108

(d) Capacity in Circuit 112

(e) Power in A. C. Circuits 114

(f ) Alternating Current Measuring Instruments 115

(g) Voltage in A. C. Circuits in Series 118

(h) Current in A. C. Circuits in Parallel 119

(i) Two-Phase and Three-Phase 120

CHAPTER VII

ALTERNATING CURRENT MACHINERY

(a) A. C. Generators 124

(b) Voltage Regulation of the Generator 125

(d) The Compounding of Alternators 130

(e) The Synchronous Motor. 132

(f ) Operation of A. C. Generators in Parallel 136

(g) The Rotary Converter 140

(h) The Transformer 146

(i) The Induction Motor 150

( j ) Starters for Polyphase Induction Motors 157

(k) Single-Phase Induction Motors 162

(1 ) Practical Remarks on Induction Motors 167

(m) The A. C. Series Motor 169

Fig. i. — View in I46th Street Power House, New York City.

THE MOTOR AND THE DYNAMO.

CHAPTER I.

INTRODUCTION.

In 1820 Oersted discovered that a magnetic needle is affected
by the presence of an electric current. — That is, the electric cur-
rent produces a magnetic field. In 1831 Faraday discovered the
induced current. — That is, when a wire -is so moved in a magnetic
field as to cross the direction of the influence of that field, an
electro-motive force is induced in the wire so long as it is in
motion. On these two principles depends the action of the mod-
ern dynamo, or generator. And if we add to- these Ampere's
researches into the motion produced by a magnetic field on a
current-bearing wire, we shall have the underlying principle of
the modern electric motor.

An apparatus for illustrating these effects is shown in Fig. 2.

Fig. 2.

M is an electro-magnet, excited from some outside source.
The magnetic field F is indicated in the conventional way by lines
representing the so-called lines of force, whose direction coin-
cides with the directive influence of the field, and whose number
at any point is proportional to the field strength. The direction
of the lines of force outside the magnet is considered to be from
the north pole to the south pole of the magnet.

2 THK MOTOR AND THE DYNAMO

If now the wire / be moved vertically downward with a quick
motion in direction d d , there will be a deflection of the gal-
vanometer G, which will begin to oscillate toward rest about
its original position the instant that the wire stops moving. The
more rapid the motion or the more powerful the magnet, the
greater in a general way will be the deflection of the galvano-
meter. An upward motion of the wire (toward d) will cause
a deflection to the opposite side, showing that the current in / is
now in the reverse direction. If the wire / be moved con-
tinually down and up, or if it be given a rotatory motion so
that its ends trace the dotted circles, which amounts to the same
thing, there will be induced in the wire an alternating current.

The wire / corresponds to one element of the winding of a
drum-type armature — the usual form in direct-current machines.
The electro-magnet corresponds to the field magnet of the dyna-
mo. The only thing required to cause the current through the
galvanometer to be always in the same direction is the inter-
position of a pole-changer, known as the commutator.

Ampere's rule for the direction of the induced current may be
most easily expressed by Fig. 3, which represents the right hand
with the index finger, thumb, and middle finger held at right

Fig. 3. — Ampere's rule for induced current.

angles to one another. It will be seen at once that a reversal of
any one of these factors necessitates a reversal of one other.

INTRODUCTION 3

Conversely, if the wires be disconnected from the galvanometer
in Fig. 3 and attached to a source of current, a force will act
between the current in wire / and the magnetic field in a direction
perpendicular to both, that is, parallel to d d. The wire / will
therefore tend to move upward or downward according to the
direction of the current. From this it is evidert that by con-
tinually reversing the current sent into / from some outside
source, this wire / may be made to oscillate up and down across
the magnetic field. If it be fastened to the surface of a cylinder
whose ends are represented by the dotted circles, the cylinder
may be made to rotate about its central axis by changing the
direction of the current in / at every half revolution. For this
purpose a pole changer or commutator is used. The apparatus
then represents the elemental direct-current motor.

Ampere's rule for the direction of the motion of / in respect
to the field, etc., is the ,same as in the preceding case, that of the
induced current, except that the fingers of the left hand are used
instead of the right.

From Ampere's rules it can be seen that in every case, the
current, induced in a wire by moving it in any given direction
in a magnetic field is such that the force exerted between this
current and the field tends to push the wire in the reverse direc-
tion. If we consider the resistance of the wire to be zero, then
the force required to move the wire through the field is equal
and opposite to the force with which the induced current in the
wire opposes this motion. The converse is also true. This
principle follows directly from the doctrine of the conservation
of energy and is one application of what is known as Lenz's law.

In order to study quantitatively the phenomena thus far men-
tioned and to deduce the fundamental formulae of the dynamo
and the motor, a few mathematical considerations are required.
The next chapter must be devoted to this purpose.

CHAPTER II.

MATHEMATICAL PRINCIPLES.

(a) Definitions.

The space surrounding a magnet in which its influence is felt
is known as a magnetic field.

The field is usually considered as made up of lines of magnetic
force whose direction at each point is that in which the magnetic
influence tends to act.

A unit magnet pole is one of such strength that it will repel
a like pole placed at a distance of one centimeter from it in air
with the force of one dyne.1

The intensity of a magnetic field at a given point is equal
numerically to the force in dynes with which the field acts on
unit pole placed at that point. The unit of field intensity is
termed the gauss. Field intensity in air is denoted by the letter
H, in other materials by the letter B. The force with which a
field in air acts on a pole of strength m is expressed by the
formula F = mH. The relative ease with which lines of force
traverse various materials such as iron, nickel, etc., air being
the standard, is denoted by the greek letter /A, and is called the
permeability of the substance in question, so that

B = Hfi. For air then, p =J /*- "

A field of unit intensity, or of one gauss, is considered as hav-
ing one line of force per square centimeter of sectional area.

Lines of magnetic force are called maxwells, and the number
of maxwells in a field is known as the magnetic flux, and is de-
noted by the symbol <£. The number of maxwells per unit
area (of one square centimeter) is the flux density (<£) and is
equal numerically to the intensity of field H or B. Hence
<£ = $ X area of cross-section of field in square centimeters.

By Coulomb's law the force in dynes between two magnet poles
in air is equal to the product of the number of units in each pole

1 The dyne is such a force as will give to a gram mass an acceleration of one centi-
meter per second. It is the so-called absolute unit of force. The acceleration due to the
force of gravity is 980 times as great.

MATHEMATICAL PRINCIPLES

(that is the product of the pole-strengths) divided by the square
of the distance between them, and so for all materials,

T'fL

It follows from the preceding definitions that a unit mag-
net pole must create at the distance of one centimeter from it
on all sides a field of unit strength, or one having one max-
well per square centimeter area. Since the area of a sphere of
one centimeter radius is 4?r square centimeters, unit magnet pole
must have 4-n- lines of force proceeding from it. The total mag-
netic flux 3> from a pole of strength m then equals

Unit electric current may be defined as follows: When unit
current flows in one centimeter of wire in unit magnetic field,
perpendicular to the lines of force, the force between wire and
field and perpendicular to both is one dyne. [The mental picture
may be formed by a reference to Fig 3.] Hence current strength
(denoted by *) is force per unit length of wire per unit field
intensity, or

*==—— and F = i/H.
/ M

This is the absolute unit of current. The practical unit, the
ampere, is yio as large as this, so that current strength in prac-
tical units is

The absolute unit of electro-motive force is the e. m. f. in-
duced in a conductor when it cuts a magnetic field at the rate
of one line of force per second. Being a rate of cutting the flux,
it may be expressed thus

'= T orbetter « = -§-•

where d® = a small portion of the flux cut and dt = correspond-
ing small portion of time. The negative sign signifies that the
e. m. f . sends a current through the conductor in such a direction
as to demagnetize the field.

THE MOTOR AND THE DYNAMO

This unit is extremely small, it taking ios or 100,000,000 of
them to make one volt. Hence

E volts =

io8/ '

(b) The Induced Current.

To find the e. m. f . induced in a straight wire moving sidewise
across a uniform magnetic field in a direction perpendicular
to the lines of force. Let the dots in Fig. 4 represent lines of
force passing vertically through the page.

1 V*

Fig. 4-

Consider the wire of length / centimeters to be moved with
velocity v centimeters per second in the direction of the arrow. In
field of strength H (= <£) lines per square centimeter the whole
wire in time t will move vt centimeters and describe an area Ivt
and cut Hlvt lines of force = <j>lvt = <£. The rate of cutting

would be — - — — — e absolute units of e. m. f . , also e = Htv.

Now if there be a current i caused to flow in this wire by
the induced e. m. f., a force F, will act in a direction opposite
to the arrow, tending to prevent the' motion of the wire. The
rate of overcoming this force will be Fz/ ergs1 per second.

1 An erg is the absolute unit of work, being the work accomplished by a dyne acting
through the distance of one centimeter.

MATHEMATICAL PRINCIPLES 7

But ei also expresses this rate of working, current multiplied
by e. m. f . being power. Hence

F^ = ei = liHv, whence F = UH

as before. Now /H is the flux cut in each centimeter of the
motion, and if we let d be the number of centimeters moved
over, the total flux cut will be IHd = $ and Fd = i&.

But force times distance is work; hence the work of moving
the wire so as to induce in it a current i is i® ergs. Conversely,
the work done by a flux <£ in causing a wire, bearing current i,
to move so as to cut all the lines of 3> is i® ergs. This follows
from Lenz's law, the directions of motion in the two cases being
opposite.

In practical units, ei ergs per second becomes 102 — g volt

amperes or watts. Since one watt is io7 ergs per second, the
number of units of this denomination in any given power must
be multiplied by io7 to equal the number of absolute units ex-
pressing the same power.

The preceding discussion is again one aspect of Lenz's law.
The work of maintaining an induced current, except for the
electrical resistance of the conductor, consists in overcoming
the opposing force which the induced current itself sets up in
conjunction with the field. From this it is at once evident
where the power of the engine goes which drives the dynamo of
a central station. The converse of this proposition will be found
on a later page in the more detailed discussion of the motor.

It remains for the present chapter to investigate the formation
of the magnetic flux by the field coils of the dynamo machine.

Consider the magnetic field inside a long solenoid, or better, in
order to avoid the problem of the ends, consider the field within
a toroid.

Suppose the field to be uniform, and let the number of turns
of wire be denoted by N. If unit pole be carried once around
the circular axis of the toroid, its 4?r lines of magnetic force will

THE) MOTOR AND THE DYNAMO

cut N turns, inducing in them a current i. We have seen that
the work done, when a conductor carrying a current i cuts <£ lines
of force, is i® ergs. Hence, considering the electrical resistance
of the wire to be zero, the total work done in carrying unit
pole once around the circuit will be 47rNj ergs. According to
Lenz's law, this work is done against the opposing force of the

Solenoid.

Toroid.
Fig. 5.

strength of field within the toroid, due to current i, namely H.
But H is also the force in dynes between unit pole and field H.
Hence, if / centimeters is the length of the circular axis, the total
work = force X distance — HI — 4^1 ergs.

This must be the work done by current i in the coil of N turns
to maintain a strength of field H, and is known as the magneto-
motive force, M.M.F.

After the analogy of Ohm's law for the electric current, we
have for the magnetic circuit the formula

M.M.F.
Flux = $ = — r—

reluctance

If ^ be the area of the field, in the present case the area of a loop

MATHEMATICAL PRINCIPLES

of a toroid, then 3> = Hs for air and <£ = HJ/A for other sub-
stances whose permeability /* is different from unity. Hence
we may write

or if I = amperes,

47TNI

The reason for putting the formula in this form is that the

numerator is M.M.F. and the denominator -- is the reluctance

SIM

of the magnetic circuit. This reluctance, like electrical resistance,
varies directly as the length (/), inversely as the cross section
(j), and inversely as the permeability (/A), which last is in a way
similar to the conductivity of an electric conductive material.

Knowing these last mentioned quantities of a magnetic circuit,
as for instance of a dynamo machine, it is possible to compute
the number of ampere turns (NI) required to produce a given
flux

(d) Magnetization Curves.

Suppose a piece of iron, such as the ring of the toroid already

Fig. 6. — B and H curve. — Hysteresis.

used, to be initially unmagnetized, and imagine a constantly
increasing magnetizing force to be applied to it, such as the
current in the winding. If the varying values of H represent

10 THE: MOTOR AND THE DYNAMO

the intensity of field that would be produced in air by this increas-
ing M.M.F. and B represent at each instant the field intensity
produced in the iron core, then a curve plotted between H and B
would have the form O^ of Fig. 6.

Since B = /*H, and the curve is not a straight line, the per-
meability of the iron, /x, must be a varying quantity. Unlike its
electric opposite, resistivity, or specific resistance, it changes with
the flux-density of the iron. At the point s, when H has the
value H', the increment of H is no greater than the increment
of B, that is, the iron has no longer any multiplying effect on
the flux and has reached its saturation point.

If now the magnetizing current be gradually decreased to
zero, reversed and increased again in the reverse direction, the
B and H curve will return from s along line scs'. If the current
be again decreased, reversed and increased to its maximum value
the curve s'c's will result. The open space between these curves
represents <the difference between the work done in producing a
flux in the iron and the returned energy furnished by the dying
out of part of this flux. In order to demagnetize the iron com-
pletely, the current must be reversed, producing a value of H
equal to the distance from o to where the curve cuts the H axis.
This is known as the coercive force and results from the retentiv-
ity for magnetism possessed more or less by all forms of iron at
ordinary temperatures, but in the largest degree by hard steel. In
the complete cycle represented, a portion of the M.M.F. goes to
overcoming this retentivity; and in a succession of reversals, as
in the case of an alternating current, the loss of energy results in
heating the iron. Its value in ergs is represented by the area en-
closed by the curve and is known as hysteresis.

The empirical hysteresis formula of Steinmetz for various
sorts of iron is w = ^B1'6, where w is the loss an. ergs per unit
volume of iron per magnetic cycle, B is the maximum value
obtained by induction during the cycle, and y is a constant depend-
ing on the quality of iron used.

(e) The Flow of Current.

Ohm's law states that current strength is proportional directly

MATHEMATICAL PRINCIPLES II

to the e. m. f . and inversely to the resistance. In practical units
this is

volts

Amperes — — r— - .
ohms

Whenever current flows through a conductor overcoming the
resistance, it is at the expense of pressure, or there is always
present an IR drop. This is a fundamental law of all electrical
circuits. Without this IR drop in voltage no current can flow.

Since IR = E and IE = power, substituting, PR = the power
lost when a current I flows through resistance R by virtue of
pressure E. If the denominations are amperes and ohms, then
PR = watts. These lost or consumed watts go to heating the
conductor, and the number of calories of heat developed is 0.24
I2R/ , where t is the time in seconds.

The resistance of a conductor varies directly with the length
and inversely as the cross-section. It also varies with the
material and with temperature. The specific resistance of a con-
ducting material is the resistance per mil foot. A mil foot is a
wire one foot long and a circular mil in cross-section. A cir-
cular mil is the area of a circle 1/1000 mcn m diameter. The
specific resistance for hard drawn copper wire at o° C. is 9.7
ohms.

As to temperature the formula states that

R^o = R0o ( i -f at} ,

where a is known as the temperature coefficient. For copper
a = 0.0042 ; for manganin, an alloy of manganese, a = almost o.
For carbon a= minus 0.0004 (about). At 20° C., therefore,
the resistance in ohms of a copper wire is

circ. mils

CHAPTER III.

THE DYANMO MACHINE.

This chapter is devoted to a general description of the direct-

Fig. 7.— The D. C. generator.

current dynamo machine and to a detailed explanation of its

THE DYNAMO MACHINE 13

various organs, their functions, and the materials used in their
construction.

Fig. 7 represents the general arrangement of the parts of the
most common type of machine. It is a two-pole shunt-wound
dynamo, and may be used either as motor or generator
indifferently. We will consider it as a generator.

By the term shunt-wound is signified the method of exciting
the field coils, the current for them being tapped from the
machine terminals T T' by a circuit which forms a shunt, or
by-path sh, to the main line-circuit L I/. The armature of this
machine is drum-wound, its wires, or inductors, being on the
surface of a rotating iron cylinder which forms the armature-
core. These wires are carried in sixteen slots on the lateral sur-
face of the core and form by themselves a closed circuit. They
are connected at regular intervals to the eight bars of the com-
mutator C. The current is taken off from the commutator by
the brushes BB'.

By applying Ampere's rule for direction, it will be seen that
in those inductors whose section is represented thus • , the cur-
rent is flowing toward the observer; and in those represented
thus o, it is flowing from the observer.

Though the diagram shows only one inductor to a slot, it is to
be understood that there are usually several wires in each slot,
the windings of two connected slots, as 15 and 8 or 4 and n,
being repeated a definite number of times before proceeding to
the next, such a group constituting an armature-coil or winding-
element.

Starting with the commutator bars touching the upper brush
and attached to inductor (or winding-element) No. 10 or I it
will be seen that we may proceed by two paths and finally arrive
at a bar which touches the opposite brush, namely, by numbers
12, 3, 14, 5, 16, 7, also 2 and 9, or by numbers 15, 8, 13, 6, n,
4, also 9 and 2.

Proceeding thus from the negative to the positive brush, the
e. m. f. generated in each inductor or winding-element is added
to that generated in the next one connected with it, so that the

14 THE) MOTOR AND TH£ DYNAMO

effect is the same as that of two series chains of voltaic cells, the
two chains being in parallel from brush to brush.

Although in any one inductor or winding-element the current
is an alternating one, changing direction twice in each revolution
of this bi-polar machine, yet the inductors to the left of the axis
of commutation ax always generate an e. m. f. in one direction
and those to the right of ax in the other direction. The rotating
armature is thus a sort of double electric pump, in which the two
cylinders work in unison converting mechanical energy into elec-
trical energy. In a machine of 100 per cent, efficiency this con-
version of energy would be perfect according to Lenz's law.

The elementary alternating-current dynamo does not differ
from the machine just described except in two particulars.
First, the commutator is replaced by two insulated rings con-
nected each to diametrically opposite armature inductors, as
8 and 16, or 4 and 12, etc., all other inductors being connected as
in the diagram, but not directly to the rings. The other differ-
ence is that the field is excited by a different current, either from
some outside source or by aid of a special commutator for the
field current alone. Alternating-current machinery is treated in
later chapters.

We are now in a position to write the general formula for
the e. m. f. generator in a direct-current dynamo in accordance
with principles laid down in Chapter II. Let

$ = the total flux passing from any one pole to the neighbor-

ing part of the armature core, or vice versa.
p := the number of poles.

N = the total number of inductors on the lateral surface of
the armature, that is, in the slots.

p' = the number of parallel paths through the armature wind-
ings from the — to the -f- terminal of the machine.

n = the revolutions per second.

Then the generated e. m. f. in volts — E = -^-§Tr~ » because

gen. IO/>

the total flux cut by the armature windings is &p, and the flux

*k THE DYNAMO MACHINE 15

cut each second by one wire on the lateral surface of the arma-

N
ture is $pn. The number of such wires in series is — r .

Now this amount of e. m. f. is generated whether the machine
be turned by some outside agent, such as a steam engine, or
whether it rotate as a motor through the agency of current fed
into its armature, because of the field flux. In the latter case,
in order to feed this current into the armature, therefore, a
slightly higher e. m. f. is required than that generated, and in the
reverse direction, sufficient indeed to overcome the resistance of
the armature windings, brushes, brush contacts, etc., which we
may denote as Rrt. This additional e. m. f. may be designated
Ia Rrt, where Ia is the armature current. In this instance,
namely that of the motor, the generated e. m. f. is termed a
counter electro-motive force, being opposed in direction to the
e. m. f. of the line which supplies the driving power.

Similarly the e. m. f. at the terminals of a dynamo never is
as high as the generated e. m. f., when the dynamo is furnishing
current, because a part of this generated e. m. f. is used up in
forcing such load-current through the resistance of the armature,
etc., R,.

Thus the formulae for the shunt dynamo furnishing current
and for the motor are as follows : For the former the terminal
voltage E, = E, — Ia R, or

o
and for the shunt motor the line voltage E/ = E^ -}- Irt Rrt or

In order to make the formulae general for all direct cur-
rent machines, it becomes necessary to notice here the two other
ways used to excite the field. Instead of letting the field circuit
form a shunt or by-path, the full armature current may be di-
rected through the field windings, forming a series machine, or
a combination of these two methods may be used, as in the
compound machine. In the last case the shunt winding of the

THE) MOTOR AND THE DYNAMO

field is augmented by a few turns in series with the main circuit.
Fig. 8 represents the three usual methods of field excitation.

Shunt.

Series.

Compound.
Fig. 8.

Letting R^ represent the resistance of the series field windings,
the preceding formulae appear as follows for bath series and
compound machines.

For the dynamo E, = E, — I* Rrt — I« R,.
For the motor E, = E, + I, R, -f L R,.

The parts, or organs, of the direct-current dynamo to be treated

THE DYNAMO MACHINE

of in detail are as follows : frame and field cores, armature
cores, field windings, armature windings, commutator, brushes,

Fig. 9.— General Electric Co. 5<>-kilowatt generator. /

brush holders, bearings, lubricating devices, insulating materials.

(a) Frame and Field Cores.

The various shapes of dynamo frames are shown in. Figs. 10,
n, 12 and 13. The bipolar type is seldom made larger than 5
kilowatts. The particular advantages of the multipolar type may
be enumerated as follows:

i. The length of the magnetic circuit is shorter in the multi-
polar type, thus making the machine more compact and reducing
the weight.

THE MOTOR AND THE DYNAMO

2. The lower reluctance of the multipolar machine reduces the
ampere turns required for excitation, which results in a saving of
copper.

3. The number of revolutions per minute is reduced by in-

Fig. io.— Bell Motor Co. Early model. Open form.

Fig. ii. — Crocker-Wheeler Co. Enclosed form, now used by many manufacturers.

creasing the number of poles, thus rendering the generator bet-
ter adapted for direct coupling to a reciprocating engine. Fur-
thermore with the same peripheral speed the larger armature
renders the centrifugal force less. For instance, if the number

THE DYNAMO MACHINE IQ

of poles be made say four instead of two, the pole-face not be-
ing reduced in size, in order to have the same number of in-
ductors under one pole, the armature will have to be doubled in
circumference. But with the same peripheral speed, the revolu-
tions per minute would be only half as great ; and the centrifugal

force, which is — - , would also be reduced to half. This ideal

advantage, however, does not occur in practice, the gap between
the poles being necessarily larger in a multipolar machine than in

Fig. 12. -Enclosed type. Four-pole frame.

a bipolar, in order to avoid magnetic leakage. In direct-current
machines it is the custom to let the pole-faces span from 60 per
cent, to 75 per cent, of the total armature circumference. The
frame is usually cast in sections and bolted together. The pole
cores are sometimes made of laminated iron, and in some machines
they are capped by a projecting plate or "shoe."

2O THE MOTOR AND THE) DYNAMO

The material of frame and cores is chosen with reference to
its magnetic properties. The B and H curve mentioned in chap-
ter II for various sorts of iron is shown in Fig. 14.

Cast iron, although the cheapest of the varieties mentioned,
requires more ampere turns than any other to magnetize it to
the proper flux-density. Hard steel is never employed for any

Fig. 13. — Complete field frame of 600 k.w. generator. G. E. Co.

but permanent magnets, such as are used in magnetos and
electrical measuring instruments. Its hysteretic qualities render
it especially valuable for such purposes. The best material for
field cores is mild steel, and this is often employed for the frame
as well. The base of the machine, however, and remote parts of
the magnetic circuit are not infrequently made of cast iron.

THE DYNAMO MACHINE

(b) Field Windings.

The shunt field windings of a direct-current machine are de-
signed to carry from about 2 per cent, to 8 per cent, of the full
load current of the machine. The number of ampere turns re-

io 30 to So to 70 »o 90 »oa UQ

AOOQ

Fig. 14.

quired for field excitation can be determined from a formula of
Chapter II, namely.

47rNI

or NI =

. 10 -

The magnetic circuit is made up of various parts, each of
which must be separately computed, owing to the different values

22 THE MOTOR AND THE DYNAMO

of flux density commonly employed in each. For instance let
the field cores be 9f soft cast steel and their dimensions be as
follows: / = 50 centimeters, j = J>525 square centimeters,

<$ = 23,000,000 maxwells. Hence B numerically = - is ap-
proximately 15,000 units. Consulting Fig. 14 it will be seen that

for soft cast steel, when B = 15,000, H = 35 and hence /* = - ~

10 X 15,000 X —

= 430 (about). Thus NI becomes - — = 1430

4*"

ampere turns.

For the other parts of the circuit such as the field-yoke or
frame, the armature core and teeth, and the air-gap, for which
the permeability is one, individual computations have to be made.
The sum of the various ampere turns found is then the total num-
ber required, for each pair of poles. Dividing by 2 gives the
ampere turns for one field coil of the series or the shunt machine.
Magnetic leakage has not been considered in this calculation :
see page 48.

The size of wire used and the dimensions of the coil de-
pend on the allowable rise of temperature. The formula for
heat developed by an electric current, given in chapter II, is H
in calories = 0.24 I2R/, where R is the resistance of the wire in
ohms. The allowable temperature rise is 50° C. above the ordi-
nary machine-room temperature of 25° C., or a maximum tem-
perature of 75°. When this point is reached, the exposed surface
of the coil should be sufficient to radiate in the cooling breeze
furnished by the rotating armature all heat energy which would
tend to elevate the temperature above 75°. Manufacturers have
various empirical formulae for approximating the relative dimen-
sions of the spool.

The winding of the field may be round copper wire or in the
form of ribbon. The insulation is usually of cotton and occupies
from 30 per cent, to 60 per cent, of the total cross section of
the winding. The coils are usually held in shape by paper or cord,

THE DYNAMO MACHINE 23

and the surface is covered with moisture-proof varnish. See

The armature core is made of iron or steel, being a very con-
siderable part of the magnetic circuit. Since the core cuts lines
of magnetic force as well as the armature windings, in a solid
core there would be nothing but the resistance of the iron to

Fig. 15.— G. E. Co.— Field-coil of B.C. generator, 200 K.W. and above.

Fig. 16.— Edgewise field winding. Crocker-Wheeler Co.

prevent a heavy induced current from flowing longitudinally
down one side of the core and back on the other side, wasting
the energy of the machine and heating the iron. Such currents
do in a measure occur and are known as eddy, or Foucault, cur-
rents. They are, however, to a large extent prevented by build-
ing up the core of sheet iron, the laminations being at right angles
to the shaft. Their thickness in higher priced machines varies
3

24 THK MOTOR AND THE DYNAMO

from 0.014 to 0.02 inch. For insulating electrically the lamina-
tions from one another some manufacturers use varnish, others
simply depend on the oxidation of the iron surface. Because of
the continual reversal of the magnetic flux through the rotating
core, a sheet iron or steel with a low hysteretic constant y is
desirable.

Two general shapes of armature core prevail in direct-current
machines, the drum and the ring. In the original machines built
and operated by Gramme at Vienna the ring armature was used.
The disadvantage of this type is that wires on the outer sur-

Fig. 17. — Gramme ring.

face only cut the field flux, the return wires through the ring
serving merely as connectors. See Fig. 17. The advantage of
this type of armature core is the superior ventilation and cooling.
In all but the smallest machines, armature cores are now cooled
by radial ducts, and the drum-wound core is the prevailing form.
The ring-shaped core is sometimes used wound as a drum, the
laminated portion being supported on a star-shaped cast-iron
frame secured to the shaft, but in small machines the laminations
for the drum-winding set directly on the shaft. In the earlier
machines the windings were secured to the core surface by

THE; DYNAMO MACHINE; 25

wooden pins. The modern method is to insert the inductors
into lateral grooves or slots on the core surface.

Fig. 1 8 shows a number of shapes of these armature laminae
or punchings. The parts between the grooves are known as
the armature teeth. As the machine rotates, the field flux sweeps
across in tufts from tooth to tooth cutting through the inductors
in the slots. The narrowest possible air space between armature

Fig. 18.

surface and field magnet face is therefore not always the most
effective.

(d) Armature Windings.

The armature inductors must be of sufficient thickness to carry
the full load current of the machine without undue heating. They
are usually of circular cross section and cotton insulated, but
on larger machines ^and particularly in alternating-current gen-
erators they are in the form of ribbon. They are usually wound
on forms independently of the armature core, a number of turns
of wire constituting a winding-element. These elements are
then placed into position in the armature slots and properly
secured and taped. The ends are finally connected to the com-
mutator bars according to the design of the winding. The slots
are sometimes lined with strips of paper or insulating fiber
before the introduction of the wires, and in certain cases the
inductors are held in position behind a strip of insulating ma-
terial that fits in a groove in either tooth, thus covering the slot.
See Fig 18. The even distribution of the windings within
the slots is a matter of great importance, especially in high
speed machines, as any inequality renders the armature poorly

THE: MOTOR AND THE: DYNAMO

balanced and causes jarring. When all the slots are filled, the
inductors are secured firmly against displacement from centrifugal
force by means of circular bands of brass or even steel wire. Figs.
19, 20, 21, and 22 show various types of armatures.

Fig. 19.— Complete armature. Fort Wayne Electric Co.

Fig. 20. — Armature in process of winding. G. E. Co.

The succession of inductors on the armature surface, that is,
the methods of connecting the winding elements into a system,
are various; but in the closed coil type two chief forms of arm-

THE DYNAMO MACHINE 2.J

ature winding prevail. They are the lap winding and the wave

Fig. 21. — Complete A. C. armature. G. E. Co.

Fig. 22. -IyO\v speed armature. 150 r.p.m. Crocker-Wheeler Co.

winding. The open circuit type, namely that in which the ar-

28 THE; MOTOR AND THE; DYNAMO

mature windings do not in themselves make a closed circuit, oc-
curs only in arc-lighting generators, and must be treated later.
The armature depicted in Fig. 7 is of the drum type. It will be
observed that in proceeding around such an armature the al-
ternate slots only are used, the intermediate ones being left free
for the return wires of some other winding element. Thus 12
and 3 are followed, not by 13 and 4, but 14 and 5. The neces-
sity for this arrangement becomes at once apparent from Fig. 23,
which represents an attempt to wind an armature using successive
slots. It will be observed that one must pass twice around the
core to form a closed coil winding. This can be easily proven
by trying it with a ball of string.

Fig. 23.

The drum armature is a development of the ring type, the
return wires through the center of the ring being carried in-
stead to the opposite side. In the ring wound armature, every
inductor or every second or third, etc., inductor may be con-
nected to a comna£nl^itor bar, making the maximum number of
bars equal to the number of inductors or groups of inductors. In
the drum type, on the other hand, there can be only one bar for
every two inductors or bundles.

The essential differences between the lap and the wave wind-
ing for armatures may be best appreciated by a study of Fig. 24.
The brushes are diagramed on the inside of the commutator so
as not to obscure the windings.

THE DYNAMO MACHINE;

I,ap wniding.

Wave winding.
Fig. 24.

3O THE MOTOR AND THE DYNAMO

General observations applicable to both windings : —

(a) There are half as many commutator bars as there are
inductors (or bundles of conductors) on the armature surface.

(b) The space or "pitch" between two directly connected
inductors or bundles, which thus form a winding-element, is
approximately such that when the one is entering the flux from
a north pole, the other is entering the flux from the south pole,
etc. This causes oppositely directed e. m. f . to be induced in the
two sides of the winding-element. It follows that the distance
between two such inductors must be neither so small as the
width of a field pole face nor so large as to reach much beyond
corresponding points of two consecutive pole-faces. Otherwise,
the e. m. f . induced in the two sides of a winding would be in
the same direction.

(c) In the simplex winding the spacing or pitch must always
be an odd number, otherwise all the slots will not be filled.

(d) It would be perfectly possible to have either half the
number of slots in the armature surface, there being two wind-
ing-elements to a slot, or twice the number of slots, there being
then half a winding-element in each slot.

(e) It would be possible to sandwich in between the slots and
bars represented a second set of slots and bars equal in number
and carrying a second and independent winding. This would be
known as a duplex winding and the brushes would be wide
enough to cover two commutator bars instead of one. In a
similar way a triplex winding could be formed, the brushes then
covering three consecutive commutator bars. By this means
each winding would be made to carry only a half or a third of
the entire current. Such types of winding as close upon them-
selves forming a single continuous circuit are said to be singly
re-entrant. All simplex windings are so. A duplex winding
such as described, on the other hand, would be doubly re-entrant.
It could be made singly re-entrant by connecting the two dis-
tinct windings in series.

(f) The commutator's position on the shaft with reference
to the windings is immaterial so long as the bars and slots are in

THE DYNAMO MACHINE 31

proper sequence. That is, the wires extending from each com-
mutator bar to the periphera of the armature may be of equal
length, curving equally in either direction, or the one wire may
extend radially out to the nearest slot, the other being longer
and curving around to its proper slot. The appearance of the
end of the armature and the position of the brushes with respect
to the field poles will be different in the two cases.

(g) The winding may be either "right-hand" ("progressive")
or "left-hand" ("retrogressive"), according as we proceed around
the armature clockwise or counter-clockwise.

(h) The number of slots chosen (for example 22) is not
exactly divisible by the number of poles (4), in order that there
may be no synchronous vibration in e. m. f., as might be the
case if at every instant each of the four groups of inductors had
exactly the same position with reference to each of the four
poles. ,,v

In reference to the lap-winding the following observations
may be made : —

(a) The number of brushes is equal to the number of poles.
It will be seen by a study of the diagram that this number is
necessary in order that the same voltage may be developed in
each armature path. In the special case of the figure, if we pass
from a commutator bar in connection with a brush to a brush of
the opposite polarity, either 4 or 6 inductors, that is, either 2 or
3 winding-elements, connected in series, are passed over.

(b) The number of paths in parallel is then equal to the
number of poles or in simplex winding p' = p. The number of
inductors in series is therefore approximately N//>.

(c) In the diagram the forward pitch, that on the commutator
end of the armature, is 5, the backward pitch, that on the farther
end, is 7. The average pitch is therefore 6. Taking two slots to a
winding-element, as here, it will be readily seen that in the lap-
winding, any even number of slots may be used. In a duplex or
triplex winding, 2 times or 3 times such even number respectively
may be used.

32 THE MOTOR AND THE) DYNAMO

In reference to the wave-winding, the following corresponding
observations may be made : —

(a) Only two brushes are necessary. In the figure, if we
pass from a commutator bar in connection with one brush to the
brush of opposite polarity, we must pass over 10 or 12 inductors,
that is, 5 or 6 winding-elements, essentially double the number
of the lap-winding. To be sure, four brushes could be used,
connected as in the lap-winding. But since the opposite side of
the commutator is already connected with each brush through an
armature winding, the only advantage of the extra brushes would
be to divide the current passing through each brush, which would
have a tendency to reduce sparking.

(b) The number of paths in parallel through the armature is
only 2 in a simplex winding, or p' —2. The number of induc-
tors in series is therefore N/2.

(c) In the diagram, the pitch is 5, the forward and back-
ward pitches being alike, although this is not always necessarily
the case. Taking two slots to a winding-element, it will be evi-
dent that the number of slots must not be evenly divisible by
the number of poles, otherwise the winding would close in pro-
ceeding only once around the armature. It must, on the contrary,
lap over one commutator bar or fall short by one as here, thus
adding or subtracting two slots from an evenly divisible number.
Hence the number of slots = pitch X number of poles =t 2. In
a duplex or triplex winding 2 or 3 times this number would be
used.

A comparison of the two forms of winding makes it evident
that in anything above a two-pole machine, the e. m. f. will be
higher with the wave-winding, other conditions such as flux,
r. p. m., etc., being the same. In a four-pole machine the e. m. f.
would be essentially twice as much with a wave-wound as with a
lap-wound armature. For this reason the wave-winding was
formerly called series winding and the lap-winding was called
parallel winding. Traction motors are usually wave-wound.
Fig. 25 shows another form of diagram. It corresponds to the
wave-winding of Fig. 24.

THE DYNAMO MACHINE

Fig. 25.— Straight diagram of armature winding.

(e) The Cummutator.

The commutator consists of wedge-shaped bars of copper
insulated from the shaft. These bars are held in place by a
retaining ring from which they are also insulated.

Fig. 26.— Commutator. Electro Dynamic Co.

Hot forged copper is sometimes used, but cold rolled bars are
preferable, because for one reason, they can be shaped more
accurately.

The insulating material between the bars is mica, the amber
mica being preferred. The thickness varies from 0.02 to 0.06
inch. A material known as miconite, consisting of powdered
mica, formed into sheets under high pressure, is much employed.

34 THE MOTOR AND THE DYNAMO

Accuracy in the thickness of the insulation is thus more easily
secured than with mica in the natural state.

When the bars and insulators have been assembled, they are
forced tightly together either by a ring and clamping screws or
by means of hydraulic pressure. The retaining-rings are then
put in place.

The armature windings are usually attached to the commutator
bars by soldering the ends into grooves. Although there are

44Q85 ^

4489 Z

,44-QQO

-44Q84

—44663

-4467(5

Fig. 27.— Structure of a commutator. G. E. Co.

objections to this method, owing to the high temperatures reached
sometimes by commutators, yet it is more sure than the employ-
ment of clamps or screws, which are likely to work loose.

The size of the commutator, number of bars, etc., depend
directly upon the style of armature winding employed. In a
general way, however, it may be remarked that high speed
machines usually have armatures of comparatively small diam-
eter, high voltage machines may usually be detected by the com-

THE DYNAMO MACHINE 35

parative narrowness of the commutator bars and their large
number, and machines for supplying large current, such as elec-
tro-plating dynamos, usually have rather long commutators, to
permit several brushes to be placed abreast.

(f) The Brushes.

In some of the earliest types of dynamos, steel brushes in the
form of solid bars were employed, because of the low friction.
Later, brushes made of strip-copper were used and still later of
copper gauze folded into several thicknesses. These are now
employed only on low voltage machines furnishing large current,
such as plating dynamos. The lower resistance of the copper
renders it better fitted for such generators than carbon.

On the ordinary direct-current dynamos and motors, however,
graphitic carbon pressed and shaped into block form is the type
of brush universally employed. The chief advantage of this
material is that its high resistance aids to prevent sparking (see
p. 58). Besides this it keeps the commutator fairly clean, does
not wear out the copper very rapidly, and is sufficiently soft to
be readily shaped to the curved commutator surface. These
brushes are usually copper-plated where they make contact with
the brush-holders.

Carbon brushes are sometimes set radially to the commutator,
but usually at a slight angle, even in machines designed to operate
in either direction. It makes little difference whether a machine
operate with or against carbon brushes.

The area of brush-contact depends upon the current to be
carried and determines the size of brush to be employed. The
approximate current density per square inch of contact area is
from 50 amperes in no volt machines to 30 amperes in 550 volt
machines. In cases where these figures would call for a very
wide brush, several are placed abreast, thus insuring better con-
tact and more even wearing of the commutator.

(g) Brush Holders.

Types of brush-holders are represented in Figs. 28, 29 and 30.
The springs are adjustable so as to maintain the proper pressure

THE MOTOR AND THE DYNAMO

of the brushes against the commutator. A slight variation of the
pressure being often sufficient to correct the defect of sparking. It

Fig. 28. — Brush and holder. Crocker-Wheeler Co.

Fig. 29.— Brush and holder. G. E. Co.

will be observed that a flexible conductor carries the current from
the brush so that it shall not pass through the spring, which might
otherwise become heated and lose its elastic properties.

THE DYNAMO MACHINE) 37

The brush-holders are connected to a rocker, enabling the
brushes to be shifted in position around the commutator. The
rocker consists of a lever or a collar, which bears both positive
and negative brushes, and may be rotated through a few degrees

Fig. 30.-G. E. Co.

around the axis of the commutator. In small machines it is
operated by a handle, in larger ones by a wheel and gearing.
When in the proper position, it may be clamped. In interpolar
variable speed motors, when the rocker has been adjusted in the
factory, it is fixed in position by pins.

(h) The Bearings.
Fig. 31 shows an approved type of bearing, consisting of a

Fig. 31. — Bearing housing open, showing bearings, oil wells, and oil rings.
Westinghouse Co.

brass or bronze collar set in Babbitt metal in the iron support.

38 THE: MOTOR AND THE; DYNAMO

The figure also shows the oil-rings used for lubricating. They
rest upon the shaft, and the oil is contained in wells through
which they rotate.

Recently ball-bearings have been employed in small machines
with great success. They require little attention and the lubri-
cant is applied only when they are first set up or cleaned.

CHAPTER IV.

OPERATION AND CHARACTERISTICS OF THE
D. C. DYNAMO.

(a) Preliminary Tests.

Before starting a dynamo or motor which has not been recently
operated, it is desirable to make an inspection embracing the fol-
lowing points.

(1) Does the armature turn easily with perfect clearance?

(2) Is the commutator clean? If not, clean it with fine sand-
paper. Do not use emery paper, as this is likely to leave con-
ducting material bridging across the insulation between the bars.

(3) In case the commutator surface is much worn by the
brushes, these cannot be fitted properly to the surface; and the
machine will have a tendency to spark when loaded. In this
event, remove one of the bearings, take out the armature, and
have the commutator turned down in a lathe. Great care must be
taken not to injure the inductors on the armature surface. When
replacing the armature, the oil rings will have to be lifted into
position.

(4) The commutator surface being even and clean, ascertain
whether the contact surface of every brush is smooth and fits
the commutator perfectly. If not, lift the brush, insert a strip
of sandpaper beneath it with the cutting side against the brush,
and draw the paper back and forth. In this way the brush may
be made to conform perfectly to the shape of the commutator.

(5) Ascertain that the brushes are set by the rocker ap-
proximately in their neutral position. If the armature wind-
ings are visible, the neutral commutator bars can be ascertained
as those immediately connected to inductors that are midway be-
tween two pole-faces. In case the armature is covered, the
neutral position can often be approximately determined as the
middle of the space allowed for play of the rocker.

(6) See that a moderate and yet sufficient pressure is exerted
4

4O THE; MOTOR AND THE DYNAMO

by the springs upon the brushes. A little experience will enable
an operator to judge of this quite accurately.

(7) Is there sufficient oil in the bearings or oil-cups?

(8) Do all electrical connections within the machine appear to
be correct? Whether or not they are correct cannot always
be determined without operating the machine, as will be described
presently. In the case of the motor, particular care must be ex-
ercised to see that the field connections are in good shape and that
the shunt field circuit through its controlling rheostat (resistance
coils) is perfect. Should the shunt field circuit become open
when the motor is in operation, the machine may be destroyed.

(9) Before starting to operate the shunt dynamo, it should
be ascertained that the external circuit is either open or not set
for excessive load, otherwise the machine will not build up, and
that the field rheostat is turned to the point of highest resistance.

(10) Before closing the switch feeding a motor, it should be
ascertained that the handle of the starting box or controller is
in the proper starting position, and in the case of a shunt motor
that the field rheostat is turned to the point of lowest resistance.

After everything has apparently been put in order up to this
point and the dynamo has been started and is being driven at its
rated speed, if it fails to build up, even when the field rheostat
is turned to the point of lowest resistance, and the switch to the
load circuit is open, this may be due to any one of several causes.
It is an easy matter to enumerate a long list of so-called diseases
of the dynamo. Experience, however, suggests the following
methods of procedure, taking each in turn until the fault is dis-
covered.

(n) Shift the brushes slightly forward and backward by
means of the rocker, watching the voltmeter meanwhile to observe
any tendency toward building up.

(12) Slightly increase the pressure on the brushes, which in
low-voltage machines may be done with the hand, watching the
voltmeter meanwhile. This may be combined with paragraph n,
and sometimes reveals a faulty brush.

(13) If the machine still fails to build and the voltmeter leads

OPERATION AND CHARACTERISTICS OF THE D. C. DYNAMO 4!

are assuredly connected and the right way around showing a
readable voltage, then open the shunt-field circuit. If this causes
a slight increase in voltage, it is a sign that the current in the
field coils caused by the residual magnetism is in a direction such
as to reduce this magnetism rather than to build it up. The
leads from armature to shunt-field must therefore be reversed.
Or the fault may be corrected by driving the dynamo in the
reverse direction, which will reverse the polarity of the armature
terminals.

(14) If operation No. 13 causes the voltmeter needle to drop
slightly, it is a sign that the field is correctly connected to the
armature terminals and the fault lies elsewhere, possibly in the
field-coils themselves. Before testing out these, however, it
would be well, slightly to increase the speed of the machine, if
this is not too difficult. A slipping belt or a badly governed
prime-mover is sometimes the sole cause of annoyance.

(15) If operation No. 13 causes no change in the voltmeter
reading, it is probable that no spark appears on closing and open-
ing the field circuit, and that the circuit is somewhere broken,
except in the case to which- paragraph 16 applies. It is not im-
possible to get the effect noted in this paragraph, even with the
presence of a spark on opening the field circuit, if just half of
the field coils should happen to be connected the wrong way
around, giving the wrong polarity to half of the field poles. This,
however, is not at all likely unless the field has been taken apart
and reassembled.

(16) It sometimes happens that the residual magnetism of the
field iron disappears or becomes reversed. In the latter case,
the machine would operate perfectly, the polarity of the ter-
minals alone being reversed, making the switch-board meters
read backwards. In the case of lost magnetism, however, the
voltmeter would show no voltage on operation. In that event
paragraphs 13 to 15 would not apply. Disconnect one armature
lead from the shunt field so that the only path between the ter-
minals of the field will be through the field coils and rheostat,
and by means of wires from some outside source of similar

42 THE MOTOR AND THE DYNAMO

voltage to that generated by the machine itself send a current
through the coils for a few seconds, so as to excite the field and
restore the residual magnetism. The direction of this current
will determine the future polarity of the machine, but will not
otherwise affect its operation. In machines of say over 25 kilo-
watts capacity great care must be exercised in opening the shunt-
field circuit when fully excited, as the inductive voltage so pro-
duced is likely to pierce the insulation. This danger can be
avoided by short-circuiting the field through a rheostat before
removing the wires of the charging current. In case the dynamo
is one of 220 or 550 volts, sufficient residual magnetism may
usually be induced in the field by a no volt source of supply, if
the higher voltage is not at hand. The utter loss of residual
magnetism by the field iron is not a frequent source of trouble.

If the dynamo still fails to build up, the trouble is likely of a
serious nature, such as to require the rewinding of field or arma-
ture. As a means of locating these more serious faults, the fol-
lowing tests are described. Paragraphs 17, 18 and 19, however,
might well be considered a part of the original inspection of a
dynamo and the test should be made on new machines or those
which have been recently taken apart and reassembled.

(17) Test for "grounds" (a) between field and frame and (b)
between armature and core. For these tests connect the volt-
meter and machine to a source of supply of similar voltage to
that of the machine, according to Fig. 32.

In case the insulation has been rubbed off one of the windings
at any point so that the bare wire lies against the iron, forming
what is known as a "grounding" of the wire, a voltmeter con-
nected as shown on a no volt circuit would read no volts. If
the voltmeter reads anything less than the circuit voltage, only a
partial "ground" is indicated. The resistance of the wire cover-
ing not being infinite, the voltmeter will always read something,
sometimes so little, however, that a special low-voltage meter is
required to get an exact reading. If the resistance of the meter
be known (Rw), the resistance of the ground (R^) may be
found as follows :

OPERATION AND CHARACTERISTICS OF THE D. C. DYNAMO

V/ = line voltage, or voltage of the source, and let V
the voltage read. Since the deflection on the scale is proportional
to the current passing through the meter, the instrument becomes
in this usage an ammeter, and we may write

V, V,

whence

and

Rw ' B

>,» 4- R^

VV,

— nr

R,« *

U. 4- R,

V,RW = VR,«

+ VR,,

The insulation between field and frame and between armature
and core varies greatly with the size and type of machine and

(a)

Fig. 32.

with the particular voltage for which it is wound. According
to the standard rating of the American Institute of Electrical
Engineers (1902) "The insulation resistance of the complete
apparatus must be such that the rated voltage of the apparatus
will not send more than 1/1,000,000 of the full load current, at
the rated terminal voltage, through the insulation. Where the
value found in this way exceeds i megohm, i megohm is suffi-

44 THE MOTOR AND THE DYNAMO

cient." Substituting this value in the formula, if the circuit
voltage is no and the voltmeter have a resistance of 20,000 ohms,

V R

a normal value, then since V = - we should have

(Rm 4- Kf)

T T O NX' 2 O OOO

V = - - = 2.15 volts. So the voltmeter on a no volt

1,020,000

machine could read 2.15 volts sum total in the tests just described
and the machine still come within the rating.

If a serious ground is discovered in any machine, perhaps the
first point of suspicion is the binding posts. It may also be sug-
gested here that by separating the field coils from one another,
each may be tested separately. Should a serious ground be
detected at two different points, this would mean that part of
the windings are made inoperative, a large part of the current
naturally flowing by the path of low resistance, namely, the
grounds.

(18) Taking the resistance of the field winding sometimes
reveals a fault, and in any event is a desirable preliminary test
on any machine. It can be easily determined by taking the poten-
tial difference between field terminals with the voltmeter when a
known current is flowing through the coils. By baring the con-
nections between the spools, the drop across each individual
spool may easily be obtained. In case any spool gives a markedly
different reading from the others, it is a sign that the winding
has been injured at some point, causing either an excessively high
resistance or a short circuit within the spool. The resistance of
the field coils can be more accurately determined by means of the
Wheatstone bridge, and the current used will not be sufficient to
heat the windings and change their resistance during the test.

(19) Taking the armature resistance is also a desirable pre-
liminary test on any dynamo machine. The best method to
employ is the potential difference method as above. The arma-
ture resistance is made up of three essential parts, namely, the
resistances of the armature winding, of the brush contacts and
of the leads to the machine terminals. The last are very small

OPERATION AND CHARACTERISTICS OF THE D. C. DYNAMO 45

and may be included in that of the brush contacts. The method
of obtaining these values is indicated in Fig. 33.

VARIABLE RESISTANCE

VWN/WV

TOTAL DROP

Fig. 33-

.00 ,30 #0

Fig. 34.— Carbon brush resistance curve.

The total armature drop ought to be small, as the armature
resistance materially affects the voltage at the terminals and the

THE) MOTOR AND THE DYNAMO

efficiency of the machine. The brush drop, the sum of the two
sides, varies from about 1.2 to about 2.8 volts. It is greatly
influenced by the strength of the current. The accompanying
curve (Fig. 34) was obtained on a 3 horse-power interpole motor
of the Electro-Dynamo Co. The current used in obtaining the
resistance of the armature circuit should be as near as may be
to the full-load current of the machine. The armature must
not be allowed to rotate as, the field not being excited, the
machine might attain a dangerous degree of speed.

This test may be extended in the following way in order to
reveal any defect in the armature winding. Hold one of the
voltmeter leads on a commutator bar in contact with a brush
and keeping the current constant, touch the other lead to each
succeeding bar in turn. The increments in voltage drop should
be constant. Any increment less than the constant indicates a
short circuit in the corresponding winding-element. A zero
increment may also indicate an open circuit in the armature.

(b) The Building-up Curve.

If a shunt generator with the field circuit open be brought up

Field current

Pig- 35.— Building-up curve.

to constant speed and the field circuit be then closed, the ter-

OPERATION AND CHARACTERISTICS OF THE D. C. DYNAMO 4/

minal voltage of the machine and the field current as the gen-
erator builds up will be so related as to give the curve of Fig. 35.
The shape of this curve depends upon the iron of the magnetic
circuit. The saturation point is at S. Owing to the hysteresis
of the iron, the time required for building up varies from a few
seconds in small machines to a minute or so in very large ones.
The curve must be plotted from simultaneous readings.

This curve, showing also the character of the magnetic circuit
of the dynamo machine, is easier to obtain than the preceding.
The method is to excite the field from some outside source, a
variable rheostat controlling the strength of the exciting current,
and to read the voltage developed at the terminals at each value

of the field. Then, since & = - —r , in which I is varied at

10 —
sp

will and tr^e only other variable is /*, depending on the flux
density induced in the iron, it follows that the m. m. f ., which is

— - , varies as I. Since m. m. f. is also HI, the field current
10

I plotted as abscissae is proportional to H. (See Chapter II.)

.
Again since e. m. f . = — ^r, — in any dynamo machine, and since

for any one machine driven at constant speed the only variable
in the second member of this equation is $, it follows that the
terminal voltage varies as 3>. Now since <£ = <J>s, or Bs, when
we plot the e. m. f.'s as 'ordinates, we are plotting values pro-
portional to B. Hence the curve described is essentially a B and
H curve, and the only reason it is not a straight line is because of
the varying values of /*. See Fig. .36.

The lower curve is drawn with gradually increasing values of
I, the upper one with the values of I again decreasing. The two
curves are not identical because of hysteresis. The proper work-
ing part of the curve during the operation of the machine is
somewhat below the saturation point, say about the region no v.

THE MOTOR AND THE DYNAMO

It would not be economical of iron to operate the machine much
below this region besides other disadvantages of having the iron
poorly saturated, to be explained later. To operate the machine

. ra . '2o

/?/7?f>erfs

Fig. 36.

much above the region would mean poor control of the voltage
generated, even if greatly varying the field current.

There is one flaw in the reasoning of the preceding para-
graph, and the curve is not strictly a magnetic curve. This

OPERATION AND CHARACTERISTICS OF THE D. C. DYNAMO 49

is because of magnetic leakage, by which is meant the stray
field extending from pole to pole out around the armature and
not ever cut, therefore, by the armature inductors. The ratio
of the total flux set up by the field winding to that part of
it which actually passes through the armature iron is known as
the coefficient of magnetic leakage; 1.25 might be given as a
normal value for this quantity, although in small machines it may
exceed this and in very large multipolar machines it is often
less.

(d) Armature Reaction.

In a generator furnishing current and in a motor under opera-
tion another factor enters in to interfere with the flux produced
by the field circuit. This is the magnetic flux due to the current
in the armature. This flux may be divided into two com-
ponents: the one (A) is caused by the cross magnetization of
the armature, distorting the field flux; and the other (B), is
caused by the so-called back ampere turns in the armature cir-
cuit and may be termed a reverse magnetization by the armature
circuit, weakening the flux from the field.

Figs. 37 and 38 show how these two effects are brought about
on bipolar machines.

Fig 37.— Generator.

The part of the armature winding represented in Fig. 37 (A)
sets up a cross magnetization, much as if the wires formed a

5O THE MOTOR AND THE DYNAMO

continuous vertical helix, the north pole being on the lower
side of the armature, at n. If the magnitude and direction
of the field flux be indicated by arrow F (N to S in the air), and
of the armature flux by arrow / (s to n in the iron), then R
will represent the direction of the resultant lines of force, the
field flux being to this extent distorted. The axis of commuta-
tion, being at right angles to this resultant flux, is given a lead
from the line of symmetry between the pole faces in the direction
of rotation by the amount 0, the angle of lead. This lead given
to the brushes causes the current in the remaining armature
inductors to be as shown in Fig. 37 (B). The current in these
inductors creates a magnetic flux much as if the wires formed a
continuous horizontal helix, the north pole being on the right
side of the armature at n. In this case the arrows F and / are
directly opposed to each other, the result being a weakening of
the field flux.

In Ampere's rule of directions, the right hand applies to the
generator and the left hand to the motor. Effect (A) in the
motor is therefore just the reverse of the generator as is shown
in Fig. 38, but effect (B) is the same in both machines. In the

(B)

Fig 38. —Motor.

motor the brushes are given a lagging position with reference
to the axis of symmetry in order to bring them into the neutral
position.

The final shape of the field flux due to the influence on it

OPERATION AND CHARACTERISTICS OF THE D. C. DYNAMO 51

or armature reaction is shown in Fig. 39. It will be observed
that the tendency is for the flux density to be increased in the
trailing pole-tip of the generator and in the leading pole-tip of
the motor.

Motor. Generator.

Fig- 39-

In designing a machine, the weakening of the flux caused by
armature reaction has to be compensated for by an increase in
the ampere turns of the field.

(e) External Characteristics.

Let a shunt generator be driven at constant speed with the
field rheostat fixed in some definite position, and let load be grad-
ually added to the external circuit by means of a bank of lamps
in parallel or by a variable rheostat. As the external current is
increased, the voltage at the machine terminals will be found to
fall. This is due to three causes.

(1) The IR drop in the armature nnnding. As the load cur-
rent I increases, the e. m. f. required to send it through the con-
stant armature resistance R increases in the same ratio. This
e. m. f. is used up in the armature and is subtracted from the
generated volts thereby rendering the terminal volts less.

(2) The consequent weakening of the shunt field current.
With fixed field rheostat the resistance of the field circuit is es-
sentially constant, and a weakened terminal voltage sends through
il a correspondingly weaker current. This lessens the flux cut
by the armature inductors, causing a still further voltage-de-
crease. After each addition of load, this interaction is set up and
continues till a balance is obtained.

(3) The Armature Reaction. It has been shown in the pre-

THE) MOTOR AND THE) DYNAMO

ceding section (d) how this phenomenon decreases the field
flux, an effect which is almost immediately followed by a fall
in terminal voltage.

Fig. 40 is the external characteristic of a 2 horse-power 110-
volt Bell motor, operated as a generator, the curve being plotted
between the load current and the terminal voltages. The part of
the curve which returns toward the origin is formed when the de-

. LOUIS CO. .New York

Fig. 40.— External characteristic of a shunt generator. Curve A, at rated speed and
field current. B, at rated speed and increased field resistance. C, at increased
speed and still greater field resistance.

creased resistance in the external circuit so reduces the terminal
voltage that a weaker rather than a stronger current flows through
this decreased resistance. This phenomenon is termed in power-
house vernacular "lying down." It sets in with a very unsteady
condition of the two meters and is accompanied by violent spark-
ing at the brushes.

OPERATION AND CHARACTERISTICS OF THE D. C. DYNAMO 53

(f) Armature Characteristics.
In the practical operation of a shunt dynamo, a decrease in

Fig. 41.— Armature characteristics of a shunt generator.

terminal voltage due to load can up to a certain limit of output
be compensated for by a decrease in the resistance of the shunt

THE: MOTOR AND THE: DYNAMO

field rheostat. This increases the field current and strengthens
the field flux to such a degree as to overcome the tendency of
the terminal voltage to decrease. A curve plotted between load
currents and field currents when the speed and terminal voltage
are kept constant is known as the armature characteristic. For
the generator of the preceding section operated at no volts and
again at 115 volts constant e. m. f ., the armature characteristics
are shown in Fig. 41.

(g) The Compound Generator.

From the external characteristic of a shunt generator, it is
at once apparent that in order for such a machine to furnish
constant voltage under varying loads, the services of an attend-
ant would be constantly in demand. For this reason various auto-
matic field regulating devices have been invented. By far the

Atnp

Fig. 42.— External characteristics of compound generators.

simplest and most practical of all these is that by which the vary-
ing load-current is carried around the field poles on what is known
as the series field winding. This varying load-current then adds
its m. m. f. to that of the shunt field current, thus producing
an increase of flux with increase of load. By having in these
series field coils the proper number of turns, the terminal e. m. f.

OPERATION AND CHARACTERISTICS OF THE D. C. DYNAMO 55

of the machine may be kept constant or may be made to in-
crease slightly or may be allowed to decrease slightly with in-
crease of load on the machine. Thus we have flat compounding
or over compounding or under compounding of the generator.
A simple calculation will make this clear, as follows:

From an inspection of Fig. 41 it will be seen that for a load
of 16 amperes the shunt field current must be increased from
0.85 ampere at no load to 1.28 amperes, an addition of 0.43
ampere, in order to mantain constant terminal voltage. If this

Fig. 43. — Short shunt.

Fig. 44.— lyong shunt.

particular field winding has in it 4,000 turns, this means an
increase of 4,000 X 0.43 = 1,720 ampere turns, or the load
current of 16 amperes must encircle the poles 1,720 -^ 16 = 108
times to produce the required additional flux, the shunt field
remaining constant at 0.85 ampere. For a bipolar machine this
means 54 turns to each pole in the series winding. Fig. 42 rep-
resents the external characteristics of a compound generator
with three different values of series field winding.
5

56 THE) MOTOR AND THE) DYNAMO

In order to overcome the IR drop that always occurs in line
wires of any considerable length, it is the custom to build gen-
erators of the over compounded type, and then by inserting a
cross shunt of the proper size at a (see Fig. 43) to regulate
them to the desired degree of compounding. There are two styles
of connecting the shunt field, termed short shunt and long shunt.

8 ShunT t-id'J winding .
C 5*«>5 field wirtctiriu
A fleulQtitn :hunf

Fig- 54- — Complete field frame of 50 k. w. generator.

There is very little difference between these two styles of connec-
tion as regards the behavior of the machine.

(h) Sparking.

The cause of sparking at the brushes of direct-current gen-
erators and motors and the means of obviating the same, par-
ticularly in the latter, have received more attention from manu-
facturers and have given origin to more types and styles of ap-
paratus than any other feature of dynamo electric machines. The
chief objections to sparking are (i) the little electric arc burns

OPERATION AND CHARACTERISTICS OF THE) D. C. DYNAMO 57

and mars the edges of the commutator bars and so increases this
form of trouble and (2) the counter e. m. f. of the little arcs
interferes with the e. m. f . of the generator and the speed of
the motor.

A study of Fig. 46 will serve to explain the cause of sparking
and the immediate remedy for it.

The figure represents part of a gramme ring armature with a
commutator bar to every second turn about the ring, the brush
being placed on the axis of symmetry (x.v) between the poles.
An application of Ampere's rule for direction reveals the course
of the generated current through the windings to either side
of this axis, as indicated by the small arrow-heads, the rotation

Fig. 46.

being clockwise. Six winding-elements, a, b, c, d, e, -f, are shown
with commutator bars i, 2, 3, 4, 5, 6, 7. Winding-element d,
being in the neutral position, has generated in it no e. m. f., the
elements to the right of it sending their current into the brush
through bar 4, and those to the left of it sending their current
into the brush through bar 5. As bar 4 leaves the brush, this cir-
cuit on the right will be broken at the brush-tip, causing an
electric arc. For when this circuit is broken, although element d
has no e. m. f. generated in it, yet it will not readily become
a path for this current from the right like a sort of side-track, as
the connections would indicate, because of self-induction.

Self-induction or inductance is a property of electric circuits

58 THE: MOTOR AND THE; DYNAMO

which tends to oppose any change in the current of the circuit.
It is particularly strong in those circuits which, because of their
helical shape or the presence of iron in their neighborhood,
naturally develop a magnetic flux when they carry a current.
Any change of this current and flux sets up a counter e. m. f.
in the circuit, which retards the change and particularly inter!" ei es
with a sudden reversal of the current. Now winding-element
d has a moment before been in position e and had a current flow-
ing in it from the left. It cannot therefore instantly take up the
current from the right and so divert the flow entering the brush-
tip through commutator bar 4.

Two things may be done, however, to remedy this difficulty,
as follows: First, the brush may be made of high resistance
material (carbon) which will aid the narrowing contact between
brush and bar 4 to oppose the flow of current by this path and
will further reduce to a minimum any current circulating around
through the brush and the short-circuited element d. Secondly,
if the poles be moved to the position of the dotted lines, or which
is the same thing, if the brush be shifted slightly in the direc-
tion of the rotation, the winding-element at d will come under
the influence of the next pole earlier than before, and the flux
from this pole will generate in it an e. m. f. which will aid
in reversing the current from the left so as to offer an un-
obstructed path to the current from the right. This current
will thus be made to enter the brush partly or wholly from be-
hind, through commutator-bar 5, thus obviating any arc between
the tip of the brush and bar 4. More will be said under the sub-
ject of variable speed motors of the means employed for creating
flux for reversal of the current in the short-circuited armature
coil. The theory and remedy of sparking is identical in the case
of the gramme ring and the drum-wound armature.

(i) Operation of D. C. Shunt Generators in Parallel.

All dynamo-electric machinery operates with greatest effi-
ciency near its point of full load, the efficiency being lowest
under light loads. For this reason, in generating plants where
the load is subject to wide variations, it is more economical to

OPERATION AND CHARACTERISTICS OF THE D. C. DYNAMO

have several smaller machines which can be run one or more at
a time than to have one large machine which for several hours a
day would be loaded to only a small part of its capacity. Hence
the necessity of parallel operation.

Fig. 47 shows the connections for a pair of shunt generators
feeding the bus bars of a distributing service. When the two
machines are running, the switches shown in the figure being
closed, the load may be distributed between the machines by the

To PRIME
MOVER

Fig. 47.— Shunt generators in parallel.

manipulation of the field rheostats. Being in parallel, the ter-
minal voltage of the two machines will necessarily be the same,
but a change of field which would cause an increase in the e. m. f.
of one results in its taking on more of the load. Its ammeter will
show that it is furnishing more current, and the load on the other
machine may be so reduced that its ammeter will read less than
zero, signifying that this generator is now drawing current from
the bus bars and is being driven as a motor, its prime mover
acting with a tendency to race. Such a state of things, should
it occur in a generating plant, would be more or less dangerous

THE) MOTOR AND THE DYNAMO

to the machinery, there being a tendency on the part of some
generators to become overloaded and on the part of those
driven to spark violently at the brushes. For the brushes would
not be in the correct motor position. The opening of a generator
field circuit would cause such a condition, and very easily, since
a shunt generator operates in the same direction as a motor, when
the field and armature connections are unchanged.

If generators are to operate together in parallel, it is desir-
able that their external characteristics should be similar. Fig.
48 represents the external characteristics of two shunt generators
that differ considerably. For convenience the current values are

Volts

^=^

•

mtmmmmmmHmm

• ~

^•••••^••••i

••••••••••^

NO"""

^-^

— — -— i~i

•^

too

00 A<

o i<

A*wp^

.*,'

) H

* a g

0 9<

Fig. 48-

plotted in opposite directions. Suppose the total load to be 600
ampers at no volts, the field rheostats of the two generators
having been adjusted so that each furnishes half. Let the total
load be reduced to 300 amperes, the field rheostats being un-
altered. The result will be a rise in voltage to 113 volts; but the
external characteristics of the machines being unlike, the points
on the curves coresponding to the new load and voltage show
one machine furnishing twice as much current as the other, 200
amperes to 100 amperes. Should the load be reduced to zero,
the voltage would evidently rise to about 117 at the point where
the curves cross, and generator number 2 would furnish about

OPERATION AND CHARACTERISTICS OF THE D. C. DYNAMO 6l

100 amperes to generator number i, driving it as a motor. Hence
such dissimilar machines, when operating in parallel under a vary-
ing total load, would require constant attention at their field
rheostats.

In a power plant when one of the generators is to be removed
from service, the procedure is to reduce its field till its ammeter
reads zero. The switch can then be opened without in any way
changing the loads on the other machines. Similarly, to bring an
idle machine into service, start its prime mover, bring it up to rated
speed, increase its voltage to that of the bus bars. (The volt-
meter of the switch-board is made to serve for any machine de-
sired by a rotating switch shown in the figure.) The switch
can then be closed connecting it to the bars, and its field can be
adjusted until it takes its share of the load. The machine switches
are not intended to be opened when any current is flowing through
them or to be closed when conditions are such that current would
immediately flow through them.

( j) Operation of D. C. Compound Generators in Parallel.

Except for the peculiar action of the series fields, the operation
of compound generators in parallel is similar to that of shunt
generators. The connections for two machines are represented
in Fig. 49.

Consider first the machines to be in operation without the
equalizer bus, only the outside blades of the three-pole switches
being closed, and consider G^ by a slight increase in speed
momentarily to take on more load than G2. The result will be
to increase the series field of G! above that of G2, which results
in increasing the e. m. f., thus still further increasing the load on
G±. The load on G2 being thus decreased, its series field is weak-
ened, thereby accentuating this effect till G2 is actually driven as
a motor. And not only so, but because of the reversed direction
of the current in its series field, G2 acts as a differentially wound
motor (see p. 77), increasing in speed the more current it draws.

Two compound generators in parallel are thus in unstable
equilibrium. By the introduction of the equalizer bus, however,
the current through the series fields becomes the same in all

THE MOTOR AND THE) DYNAMO

machines so connected, the resistances of the series fields being
the same. It is therefore impossible for the phenomenon just
described to take place. It is only by the introduction of this
equalizer connection, Fig. 49, that compound generators can be

SERVICE Bus.

EQUALIZER Bus.

SERVICE Bus.

To PRIME ~
MOVER

To PRIME

MOVER2

SH. F.
Fig. 49.— Compound generators in parallel.

operated in parallel. If the machines are not of the same size,
the series field resistances must be in proper proportion.

(k) D. C. Generators in Series.

Direct-current generators are operated in series for the same
purpose that batteries are joined in series, namely, to obtain
increased voltage. There are three applications of this method
of operation in general use : viz., in the Edison three-^wire system,
in the use of boosters, and in the multi-voltage power systems.

The Edison three-wire system is a device for saving copper in
transmission lines. The connections are shown in Fig. 50.

The current in the middle wire is at every point the algebraic
sum of the current in the outside wires. The saving in copper.

• OPERATION AND CHARACTERISTICS OF THE D. C. DYNAMO 63

if all three wires are of the same size, is 62^/2 per cent., proved
as follows. Double the voltage gives the same power in watts
with half the current. The PR loss in the conductors willf be
the same on a 220 volt system as on a no volt system, if R be
made 4 times as great, or the cross-section of the wire % as
great. On a two- wire system this would mean a saving of 75
per cent, of copper in a 220 volt system as against a no volt
system, but a third wire of the same size as the other two adds to
the 25 per cent, used half again as much copper, or in all 37^
per cent., leaving the economy in copper a 62l/2 per cent, saving.

ifO A 30 A 3.0A 10 A

OK) (SlO QlO QIC

I I -» *-| 2-1

10 A

30 A iOA 10 A

Fig. 50. — Edison 3-wire system.

In connection with the Edison system the three-wire generator
ought to receive a mention. This is a 220 volt generator, whose
brushes are connected to the two outside wires of the system.
Besides the commutator, there are on the armature shaft two
rings, tapped on to the winding as. in a single phase alternating
current generator. These are connected through a highly induc-
tive circuit, known as a reactor, whose middle point leads out to
the middle wire of the system. The current (direct-current) in
this wire flows alternately through either half of the reactor
circuit, thus forming part of the alternating current in the
armature.

Boosters are low-voltage generators of large current capacity
used in series with the main generator of a system for the pur-
pose of stepping up the voltage a few points on special branches
of the main system. For instance, where storage batteries are
in use to operate in parallel with a generator, as is sometimes
done in systems where there is great variation in load, during

64 THE) MOTOR AND THE DYNAMO

those hours of the day when the load is light and the battery is
being charged, a voltage above generator voltage must be applied
to the battery terminals in order to overcome the counter electro-
motive force of the battery. The additional volts would be
obtained by means of a booster. Again in order to counteract the
IR drop in line wires, the feeder system as illustrated in Fig. 51
is very efficient and convenient.

feeder

Booster

Fig. 51.— Booster-feeder system.

The booster need be only a fraction of the size of the main
generator, as it supplies only a few additional volts to part of
the load.

Multi-voltage power systems will be noticed under the subject
of variable speed motors.

(1) D. C. Arc-light Dynamos.

The open arc-light operates best at 45 volts and requires from
6.5 to 10 amperes. Because of their low voltage and large cur-
rent such lamps came to be arranged in series groups, and
machines were devised which furnished constant current at a
high voltage in distinction from the constant voltage generators
for incandescent lighting service. These machines, however, are
now rapidly passing out of use for two reasons: First, the
enclosed arc lamp has been invented, which requires about 75
volts at the arc and takes 5 amperes, more or less, so that with a
small rheostat in series with each lamp, these lamps operate very
well in parallel on the usual no volt circuit. Secondly, where
for any reason series lamps are preferred, alternating-current
service has so many advantages over direct-current for series
lighting, that the latter is being rapidly superceded. Since, how-

OPERATION AND CHARACTERISTICS OF THE D. C. DYNAMO 65

ever, such circuits are still occasionally met with, it may be best
to introduce here a brief description of some types of constant
current high potential direct-current generators.

Since the lamps are in series, when a lamp is shunted out of
service, in order to maintain constant currents, the generator volt-
age must be decreased, and vice versa. Two ways of decreasing
voltage are to- decrease the field circuit by means of a rheostat
or to shift the brushes so as to include fewer active armature
coils between them. Either of these methods used alone causes
violent sparking at the brushes, as is the case in the old Thomson-
Houston dynamo which employs the second method. The
Excelsior arc-lighting generator and the Brush machine use
both methods combined and are more successful. In all these
machines the field regulation or brush shifting, as the case may
be, is accomplished by an automatic device more or less compli-
cated in construction and adjustment. A fuller description of
these will be found in such detail works as Crocker's "Electric
Lighting," Vol. I, Sheldon and Hausmann's "Dynamo Electric
Machinery," Vol. I, etc.

In conclusion it should be stated that there are numerous
variations of the types of dynamos thus far treated, some to be
found only in Europe, such as the disc dynamo, others used only
as motors, which will be taken up later. The underlying prin-
ciples of all these are not different from those described. The
greatest divergence of design occurs in the case of alternating-
current generators, which will be treated in the second part of
this volume.

CHAPTER V.

THE D. C. MOTOR.

(a) Operation and Characteristics.

The fundamental equation of the shunt motor, as given on
page 15, is

~~ "

where E/, the line voltage applied to the machine, is opposed
by the counter e. m. f., leaving only a small remnant to send the
working current Ia through the low resistance of the armature
circuit Rrt. For instance, in a certain 3 horse-power (so rated)
shunt motor, 115 volts, 25 amperes, the field current is i ampere
and the armature current at full load 24 amperes. The armature
resistance is 0.45 ohms. Now the IaRa drop = 24 X 0.45 =
10.8 volts. Hence this is the effective pressure, the potential
difference required to send 24 amperes through the armature
circuit when the machine is at rest. The c. e. m. f . developed by
the rotating armature, that is, the e. m. f., with the same field
and speed conditions which the machine would develop if opera-
ting as a generator is 115 minus 10.8, or 104.2 volts.

When the machine is running, this c. e. m. f . of 104.2 volts
acts like a resistance, preventing the current from becoming
excessive. But if the motor when at rest should be connected
directly to the 115 volt mains without any starting device, the

current in the armature would be -- = 2,555 amperes, an

°-45

amount which, if it should not open circuit-breakers or blow
fuses, would burn up the armature circuit. While a direct-
current motor is coming up to speed, therefore, a temporary
resistance is thrown in between the armature circuit and the line,
in the shape of a starting-box. The shunt field circuit, on the
other hand, has in itself such a resistance that it can be con-
nected directly to the mains without injury.

THF, D. C. MOTOR 67

The connections of the ordinary starting-box are represented
in Fig. 53.

When the switch S has been closed, the handle of the starting-
box is slowly moved from stud to stud, cutting out the resistance-
coils R, till the left-hand lead is directly connected to the arma-
ture and field like the right-hand lead. These resistance-coils,
although at first interposed in the field circuit, have no appre-

Fig. 52. — G. E. Co. 50 h-p. motor with starting-box and circuit breaker.

ciable effect on the shunt field current, because their resistance
is infinitesimal as compared to that of the field. In some boxes,
the field current does not pass through these coils at all. M is
simply a retaining magnet, or no-voltage release, for the handle.
When the motor is to be stopped, the switch S is pulled, M loses
•its magnetism, and the handle flies back by means of a spring to

THE: MOTOR AND THE; DYNAMO

Fig- 53-— Ordinary shunt-motor starting box and connections.

Fig. 54.— Starting-box for shunt motor with no-voltage release. G. E. Co.

THE D. C. MOTOR

its original position, ready to be used again. Other forms of
starting device will be treated later.

In Chapter II we had the formula W = i&, where W = ergs

Fig- 55-— Starting-box, interior. G. E. Co.

Fig. 56.— Starting-box with no-voltage and overhead release. G. E. Co.

performed when a wire, having current i absolute units, is moved
by magnetic influence so as to cut $ lines of force. This formula
can be developed so as to give an expression for the torque T

/O THE MOTOR AND THE DYNAMO

or twisting force of the armature of an electric motor as follows.
The total flux cut per revolution by each armature inductor is
the $/> of the fundamental equation of the motor, which takes
the place of the 3> in the above formula. Similarly i must be

* N

replaced by ~j- where N is the number of armature surface-
inductors, as before, and p' is the number of paths in parallel,
4 being the total armature current, Irt, in absolute units. The
work in ergs per revolution of 360° or 2ir radians (angular
measure) = 2-n-T, when T is the force in dynes operating at the
end of a radius of I centimeter. Hence

T -

To express this torque in pounds developed at the end of a
foot radius, the usual practical torque unit, the following changes
are necessary.

i ft. Amps.

<!>/>N j

, • A A . .

p 10 27T X 2-54

cm. per in.

X 453-6 X 980 X

gms. per Ib. dynes i

12 '

per gm.

or T = — ^r~j-Ia X o. 1 175 where Ia is in amps.
io/>

Now since power, P, is work per second, Wn, we have

P (in ergs per sec.) = 2vnT = -^— ia absolute.

To reduce to watts, or io7 ergs per second, and to amperes,

= volts. X Amps

3>/>Nrc 3>/>Nrc Volts f

P(m watts) = ^g X ia X io = ' ia = c.e.m.f. X L

which is simply another form of Lenz's law.

The interpretation of the equations of the motor will make
clear the characteristics of the machine. In the first place in
the equation K/ — c. e. m. f, -{- I(IRa-

E/ and R^ are essentially constant. Consider now .a load to
be thrown on the motor, as happens when it is made to drive
machinery. The decrease in speed due to the load decreases the

THE D. c. MOTOR 71

c. e. m. f ., as is evident from the formula for the same, and the
result is an increase in Ia. This means an increase in torqu
This increase in current and torque is more rapid than the accom-
panying decrease in speed, and increases automatically, the greater
the load put upon the motor. It is therefore unnecessary to
feed into a motor by rheostat control or otherwise more or less
current according to the power desired, for if the e. m. f. of the
supply mains is kept constant, the motor will draw whatever
current it needs to meet the load. In the direct-current motors
indeed it is possible to overload the machine to such a point that
the load current will overheat and destroy the armature. In
this way a motor may be made to furnish many times its rated
power, the current capacity of the windings alone determining
the limit of power.

Another thing which the motor formulae make clear is the
fact that a decrease in the field current of a shunt motor increases
the speed. This is the most common method of speed control
for such motors and is effected by means of a rheostat inserted
in the field circuit, exactly similar in many cases to the rheostat
used to control the voltage in shunt-wound generators. From

formula H/ = - I^R* it is evident that if a shunt motor

be furnishing a given torque and its field current is decreased, 3>
will be made smaller and the c. e. m. f. therefore also smaller.
This will cause an increase in Ilt and the machine will speed up.
This increased value of n will operate to counteract the decrease
in 3>, and a new balance will be obtained between the impressed
volts E/ on the one hand and the c.e.m.f. plus armature drop on
the other. If the torque remains constant throughout the opera-
tion, since P varies as nT, the new point of equilibrium will show
an increase in developed power over the old, the decrease in 4>
not being quite compensated for by the increase in n, and the
new value of Ia being therefore greater than the original value.
If, however, the torque demanded of the motor be so decreased
wth increase of speed as to keep the power constant, then the
increase in !„ will be only momentary, the new speed almost
6

72 THE MOTOR AND THE DYNAMO

exactly compensating for the decrease in <£ so that the c. e. m. f .
is kept constant.

From this discussion it will at once be evident that an increase
in the load on a shunt motor is always accompanied by a slight
decrease in speed unless special means are taken to prevent it.
For when la increases, the impressed voltage remaining constant,
the field current remains constant, and therefore the principal
agent in effecting the necessary decrease in the second member
of our equation is n. In actual operation, though the field
remain constant, 3> is not absolutely unchanged with increase of
Ia. The armature reaction, noticed under the characteristics of
generators on page 50 lessens the field flux in motors as well,
and so acts like a resistance in the field circuit. For this reason
the -speed does not fall off as much as it otherwise would with
increase of load. In fact, it is possible in some machines to set
the brushes in such a way that the speed will not decrease at
all, or may even increase. This effect would be brought about
by an extreme backward lead of the brushes. It is usually
accompanied, however, by a decrease in efficiency and by danger
of sparking, and is therefore not usually resorted to.

In the case of the series motor, where the armature and field
current are necessarily the same, the increase of load on the
machine brings with it an increase of field flux <£, and hence
a far greater decrease in n than in the case of the shunt motor.
With this increase in 3> and Ia there comes also a greater increase
in torque than in the shunt machine, for torque varies as 3> X I.
It is the peculiar characteristic of a series motor to show great
changes of speed under varying conditions of load, and at the
low speeds to develop a very high torque. For this reason series
motors are particularly adapted to purposes of traction. When
a car of any sort is starting, the torque to overcome the static
friction must be large and the speed low. After the inertia of
the mass has been overcome, the little power required to main-
tain motion on a level track or road reduces both Ia and $, hence
the great increase in -speed, n. The shunt motor, on the other
hand, is well suited to operate machinery of nearly every type,

THE D. C. MOTOR

approximately constant speed under varying loads being the
usually desired condition of operation.

A favorite method of obtaining the characteristic curves of
motors is by means of the friction brake or other form of

Fig. 57.— The friction brake.

dynamometer. Fig. 57 represents a convenient form of such an
apparatus.

W is a piece of heavy cotton webbing placed about the pulley-
wheel, as shown. B is a spring-balance for regulating the degree
of tension by means of the hand-wheel and screw S. L is a

Fig. 58-— Cross-section of wheel for brake.

steel-yard or beam-balance. The difference in reading between
L and B is the pull exerted by the motor at the rim of the wheel,

74 THE; MOTOR AND THE DYNAMO

and this number multiplied by the circumference is the work
per revolution. In getting the circumference of the wheel the
radius is taken to the middle of the strip of webbing. The wheel
itself is preferably larger than the pulley-wheel usually fur-
nished with the motor, and the surface should be flat, not
crowned. It is also well to use a wheel of special form, whose
cross-section is shown in Fig. 58. This is capable of holding
water, which may be replaced from time to time during the test,
keeping the wheel cool. This insures greater constancy of fric-
tion and prevents the heat developed by the brake from being
conveyed through the shaft into the bearings. The formula for
power developed by the motor is then

Lbs. X ft. circumference X r.p.m.
Horse-power — - - .

33,000

This is the output. The input in watts may be obtained by an
ammeter in the general circuit (field and armature) and a volt-
meter across the terminals. Horse-power input is the watts di-
vided by 746, and the per cent, efficiency is

output

-T— - X loo.
input

(b) Varieties of Field Excitation.

Fig. 59 shows the characteristic curves of a Bell Electric
Company's shunt motor, rated at 3 horse-power, 115 volts, 25
amperes 1,200 revolutions per minute. Fig. 60 shows the char-
acteristic of a General Electric Company's crane motor (series),
rated at 5 horse-power, 220 volts, 25 amperes full load.

From the formula E/ = c. e. m. f. -f- IrtR« it is evident, that
the smaller the resistance of the armature circuit, the more
nearly constant will be the speed of a shunt motor under vary-
ing loads. For the smaller the changes in IrtR.T, the more

/ 3>/>N# \
nearly will c. e. m. f. ( = — 8 , 1 approach a constant value. In

the actual operation of nearly every motor, on the other hand,
it must not be overlooked that the speed variation is consider-
ably greater than appears from the manufacturer's curves. The

THE D. C. MOTOR

reason is the IR drop is always present to a greater or less de-
gree in the live wires leading to the machine from the source
of supply. Unless the generator is compounded for this particu-

H-P -Output

Fig- 59.— Curves of a shunt-motor. Bell Electric Motor Co. 3 H. P.

lar circuit or unless some other equally efficient means is adopted
to maintain constant load-voltage, this IR drop lowers the voltage
at the motor as the load increases, and has to be reckoned with in
considering motor speeds.

THE; MOTOR AND THE; DYNAMO

Again, the speed of a motor, both shunt and series, is not the
same after it has been operating for half an hour as at first.
The heating of the field increases its resistance and so decreases

Fig. 60.— Curves of a series motor. G. E. Co. 5 H.P.

the flux. This results in a rise in speed which may be as high
as 4 or 5 per cent.

Shunt motors may be compounded like generators, the char-
acteristics of such machines resembling those of the shunt mo-
tor, but inclining toward those of the series motor in shape. An

THE D. c. MOTOR 77

interpretation of the formula of the compound motor will make
this clear, namely,

E/ = c. e. m. f. + IaRa + I,R,

where ISRS is the IR drop in the series field winding. Since Ia
and 1^ are equal, the equation may be written

E = c. e. m. f. + Ifl(Rffl+ R,),

which shows that the series field is equivalent to an added re-
sistance in the armature circuit. Hence the speed variation in
such motors is larger than in the shunt motor. But not from
this cause alone is this true. I? in a compound wound motor goes
to increasing the field flux, <£, rendering a still greater decrease
in n necessary to reduce the c. e. m. f . so as to balance the above
equation than would be required in the case of the shunt motor.
On the other hand, the increase in 3> with load increases the
torque, so that a compound motor not only starts more slowly
than the same machine would without the compound winding,
but exerts a greater torque at starting. It is therefore suited
for those cases where an essentially constant speed motor is
desired, but one that is capable of meeting the requirements
of a widely varying load. This is the case in the operation of
passenger elevators. For derricks, on the other hand, and mine-
hoists, where the speed variation is of minor importance, the
series motor is more serviceable.

Given a compound generator, Fig. 61, to be operated as a com-
pound motor, the series field connections must be reversed as
in Fig. 62, else the series current will flow in a direction to op-
pose the shunt field.

If the generator of Fig. 61 should be operated as a motor,
without reversing the series field terminals, it would be what
is known as a differential motor, the series field acting so as
to reduce the flux with increase of load. The result is a less
falling off in speed than when operated as a shunt motor, that is,
without the series field. In fact the automatic flux reduction by
this means may be sufficient to increase the speed with increase
of load. Operation of a motor under such conditions is a matter
of great risk, as the speed may rise to a dangerous degree.

7o THE MOTOR AND THE DYNAMO

Although the differential machine shows a slightly lower effi-
ciency than the others, yet it would be serviceable where abso-
lutely constant speed is demanded. The differential motor is,
however, little used.

From this discussion of the motor there ought not to be omitted
a warning against loose shunt-field connections. When a motor

Fig. 61.— Compound generator.

Fig. 62 —Compound motor.

is running free or only lightly loaded, the opening of the shunt
field circuit results in a sudden decrease of the field flux almost
to zero. The result is an inrush of current and an enormous
increase in speed, so great and sudden in fact, that unless a fuse
is blown or an automatic circuit-breaker opens in the line, the
armature will fly to pieces simply by centrifugal force. Not too
much care can therefore be taken to have all connecting points in
the current path of the shunt field winding secure.

THE D. C. MOTOR

The control of speed of a shunt motor by means of resistance
in the field circuit has been mentioned on page 71. The per-
centage decrease in field required to bring about a given increase
in speed depends on the magnetization curve of the machine
combined with armature reaction. This latter becomes greater
and greater, distorting the field flux more and more, the weaker
the flux becomes. Fig. 63 represents the distribution of flux in
a motor field (a) with strong field excitation and (b) with reduced
excitation for increase of speed, both being under condition of
full load on the motor.

Such field distortion naturally leads to sparking (see page 63)

pole

po \<

(A)

(B)

Fig. 63.

even when the brushes are shifted to the new neutral axis.
Sparking sets the limit of speed increase by this method.

The ordinary type of shunt motor with simplex-wound arma-
ture can stand a speed increase of only about 30 per cent., for
which a field decrease of about 50 per cent, is required. Numer-
ous means have been devised, however, for overcoming spark-
ing, so that now a speed range of from i to 6 is successfully
obtained in shunt motors. Underlying these various devices of
different inventors and manufacturers there are but two funda-
mental principles to be observed. The one is the reduction of
the self-induction of the armature circuit to a minimum, the
other is the prevention of the distorting effect of armature reac-
tion on the field flux.

Self-induction in an electric circuit varies as the square of the

8O THE MOTOR AND THE DYNAMO

number of turns. This will be evident if we consider two for-
mulae of Chapter II, namely,

4?rNI d&

reluctance ' at

The flux $ in this case is not the field flux, but a flux due to
the current in those armature coils in which commutation is tak-
ing place. They are the coils short-circuited by the brush. It
is the dying out and rebuilding in the reverse direction of this
flux which causes the inductive e. m. f ., and hence the spark at
commutation.

In self-induction, where the same turns of wire produce the
flux as cut the flux, a doubling of N means a quadrupling of e,
or the e. m. f. of self-induction varies as N2.

If therefore each armature winding-element be made to have
half the number of turns and the number of winding-elements
be doubled, the self-induction will be greatly lessened. This
arrangement necessitates that the commutator be made to have
twice the number of bars, so as to accommodate the increased
number of winding-elements.

Another thing that aids in decreasing self-induction is to have
the current in each armature inductor comparatively small. This
can be brought about by making the armature winding duplex
or triplex, which causes the current to be shared by two or three
windings. This again increases the number of winding-elements
and of commutator bars. It also requires a wider brush, two
and three bars being covered by the brush face, according to the
winding. To be sure, both of these features of the winding call
for a larger armature core than usual, and also a larger
commutator.

The increased width of brush necessitated by the above-men-
tioned features, lengthens the period during which the coil is
short-circuited by the brush, that is, the period of current reversal
in the coil — another aid in reducing the self-induction, as appears

from e = —rr-
at

The increased size of armature calls for a larger size of field

THE D. C. MOTOR 8l

•

than common, hence the gain in convenience of speed control is
in a measure offset by unwieldiness and expense of machine.
In general practice, almost any ordinary shunt or compound
wound motor in which the brushes are made to overlap two or
three commutator bars will be found capable of a i to 2 speed
range, if not more, by means of field rheostat control. Greater
speed ranges than this, whatever other means may be used to
prevent sparking, call for motor frames as follows : —

Power of motor. Size of frame.

3 horse-power 5 horse-power

5 horse-power 7 '/6 horse-power

10 horse-power 15 horse-power

The Bullock Mfg. Co. resorts to lengthening the armature as
well as increasing the diameter, whence the unusual size.

As regards the means of preventing the distorting effect of
armature reaction on a weak field, several manufacturers resort
to a special shape of pole-piece. The Stow motor, instead of
changing the field current, has movable iron plungers, forming
the centers of the field cores and operated by gear-wheels. An
increased air gap increases reluctance and lessens flux. A speed-
range of I to 4 is obtainable in these machines, but the gearing
is unwieldy and expensive.

In order to avoid the extreme distortion caused in a weak
field by armature reaction, the Fort Wayne motor makes use of
a divided field core. See Fig. 64. By this means the one-half

Fig. 64. — Pole piece of Fort Wayne motor.

of the field core is kept fairly saturated so that the flux from the
other half may not so readily be crowded into it by armature
reaction. The flux curve, then, is somewhat as shown in Fig. 65
under weak field.

THE; MOTOR AND THE DYNAMO

The newest type of Storey motor goes a step further in this
direction, and allows of no direct magnetic connection between
the two halves of field core, the encircling binding-ring being
made of brass.

In Fig. 38 (A) it was shown how the current in part of the
armature inductors creates a cross magnetization, distorting the
field flux. The attempt has been made to counteract this arma-

poU

I n

Fig. 65.

ture reaction by means of a compensating winding imbedded in
the pole faces and connected in series with the armature so
as to carry the armature current, only in a reverse direction.
(Fig. 66.)

Fig. 66. — Compensating winding.

This corrective device was not found sufficient to prevent
sparking with weak fields and furthermore proved expensive in
manufacture. It also interfered with cooling.

The most recent and by far the most successful device for
counteracting the effects of armature reaction is found in the
interpole motor, for whose invention credit must be given to

THE D. C. MOTOR

Mr. Pfatischer of the Electro Dynamic Co. Fig. 68 shows such
a motor of this company and Fig 69 the same machine with
armature and end-plate removed. The interpoles carry a wind-
ing of a few turns connected in series with the armature circuit.

Figs. 67.— Curves of an interpolar motor, at high speed. Electro Dynamic Co.

Their function is to inject into the short-circuited armature
inductors at the instant of commutation just the requisite flux
for reversing the current. The excitation of the interpoles, being

THE MOTOR AND THE DYNAMO

Fig. 63.— Interpole motor— commuter end. Ball bearings

Fig. 69. — Interpole motor. Complete field frame. Electro Dynamic Co.

THE; D. c. MOTOR

accomplished by the load-current, varies with the load, as it
should. The flux for reversal, being thus provided exactly where
it is required, the shunt fields may be made as weak as is desir-
able to secure proper speed and torque, without the evil effects
of armature reaction. The brushes, furthermore, are set per-

Wiring Diagram, showing electrical con-
nections between the armature,
field, and "Inter-poles."

Fig. 7°.

manently on the line of geometric symmetry between the poles,
thus enabling the machine to be operated in either direction.
When properly adjusted, it is found that the angle of lag usually
given to the brushes of shunt motors is unnecessary. This motor
also takes advantage of the large armature and commutator to

86 THE MOTOR AND THE) DYNAMO

be found in most adjustable speed shunt motors, as previously
described, thus securing a range of speed from i to 5 or even
i to 6 in either direction, without sparking.

In connection with this motor it should be noted that a very
slight shifting of the brushes from the correct position causes it
to behave in a curious way. The interpole then acts somewhat
like a differential series field, causing the machine to speed up
till the c. e. m. f . is above the line voltage, and for a moment the
motor acts as a generator, boosting the voltage of the whole
system a point or two. This immediately causes a falling off in
speed, when the same thing is repeated. This is not a frequent
phenomenon, however, where these motors are installed, and is
guarded against, after the brushes have been once adjusted, by
fixing them in position.

All means of speed control thus far considered have been for
increasing the r. p. m. For the reverse process, when the full
field current is on, there is no convenient method adapted to con-
stant speed motors. A rheostat in series with the armature,
although it would reduce the speed, is to be avoided for the
reason that variable load makes the IR drop over the rheostat
variable, which is the same thing as applying a varying voltage
to the armature, a decreasing voltage with increase of load. The
speed current curve then declines toward zero speed.

An entirely different type of multi-speed motors to those thus
far considered is one having two distinct windings on the arma-
ture and two commutators. By having the numbers of inductors
in these windings related as 2 to 3, the following relative speeds
may be obtained :

Connection. Speed.

2 and 3 opposed highest speed, 5 X a constant

2 alone 3 X a constant

3 alone 2Xa constant

2 and 3 in series lowest speed, i X a constant

A similar system to this and one which has been developed
with more success commercially is that of operating motors on
multi-voltage lines. The field is excited on the highest voltage,
and different voltages are applied to the armature, according to

THE D. C. MOTOR

the speed desired. The various voltages are obtained by dynamos
operating in series. Three such systems are in existence, and
are represented in Figs. 71, 72 and 73.

Intermediate speeds are obtainable by field control. All these
systems differ from those of the single voltage field control

115

1*7

Fig. 71.— Ward

Leonard.

Speed

ratios, 1:2:3:4:6:7.

2.1-0

100

1 J

Fig. 72. — Crocker Wheeler. Speed ratios, 1:2:3:4:5:6.

T , J

T .

Fig- 73- — Bullock. Speed ratios, 3 : 4 : 5.5 : 7 : 9 5 : 12.5.

method in the fact that with added voltage there is an added input
and added horse-power developed. In other words, these latter
systems supply constant torque with various speeds, in distinction
to constant horse-power with various speeds, which characterizes
7

88 THE MOTOR AND THE DYNAMO

the interpolar motor and its predecessors in the market. And
when it is considered that the electric motor is a machine which

.Line

Fig. 74-

in any event automatically develops torque and horse-power
according to the load put upon it, the fact that the interpolar

THE D. C. MOTOR

90 THE: MOTOR AND THE: DYNAMO

shunt motor operated on a single voltage is rapidly superceding
all other direct-current devices can be easily accounted for.

(d) Starting-boxes and Controllers.

Together with the adjustable speed motor there has come in
a new form of starting-box with a self-contained field rheostat.
Fig. 74 shows the internal connection, and Fig 75 the external
appearance of the box. The handle is double. The movement
for starting is the same as iA the ordinary box. At the end of

Front view. Back view.

Fig- 77-— Field rheostat for generators. G. E. Co.

the starting stroke the handle divides, one blade being held by
the retaining magnet, the other being movable back across the
box-face, throwing increasing resistance into the shunt-field cir-
cuit by means of the upper row of studs.

The usual form of field rheostat, used with the ordinary type
of starting-box, is shown in F'ig. 76. Its resistance coils or
strips are embedded in porcelain. The type of rheostat more
particularly used for voltage control in generators is shown in
Fig. 77.

In regard to starting-boxes for shunt and compound motors,

THE: D. c. MOTOR 91

it should be noted that the resistance coils for the armature
current are capable of carrying that current only for a short
time without overheating. The box should therefore never be
used as a speed-reducing rheostat, unless attached to a motor
much smaller than that for which it was designed. Under ordi-
nary conditions the starting-box for shunt motors is designed to
be used 15 times an hour without overheating and it is calculated
that 15 seconds may be consumed in the operation of starting.

Fig. 78.— General Electric Co. Ratchet-driven remote control rheostat.

Small motors and motors not starting under load require much
less time than this.

It may be laid down as a general rule, that a motor starting
free should begin to rotate when the starting handle is on
the first contact point of the box. Under load the handle may
be moved to the second or even the third before the armature
begins to rotate; but if the machine does not then start, it is a
sign of too heavy a load at starting or of some other trouble.
Some boxes are provided with an overload release, which is in

92 THE: MOTOR AND THE: DYNAMO

effect nothing more than an automatic cut-out or circuit-breaker.

For stopping a shunt or compound motor, the handle of the
box should never be moved back across the studs, as this will burn
and roughen them. The supply switch must be opened instead,
leaving the handle to return automatically to its first posiion.

For the series motor the starting device is simply a rheo-

Starting rheostat for series motor.

G. K. Co. reversible controller for series motor.
Fig- 79-

stat in series with the machine. Because of the heavy current
used in such machines, large starting torque being usually sought,
the close-lying knobs and light coils of the shunt starting-box are
unsuited. Fig. 79 represents a series motor controller and rheo-
stat. The contact fingers are of heavy copper and spring-hinged.
They are also separated from one another by thick insulating
partitions, usually of asbestos. An electro-magnet provides flux
for blowing out the arcs at contact points.

THE D. C. MOTOR 93

(e) Motor Uses.

Because of the great convenience and other advantages of
electric driving apparatus, most makers of machine tools and
other factory appliances to-day equip them with motors and pro-
vide places on the frames for installing the same. In their new
and comprehensive work on electric motors, Messrs.- Crocker and
Arendt enumerate many points in favor of separate electric drive
for the machines of manufacturing plants as against the older
system of overhead shafting and pulleys. Some of these are
as follows :

(1) The lo<ss of power incident to shafting and belts is pre-
vented.

(2) Better lighting and greater cleanliness are obtainable.

(3) Floor space may be utilized to better advantage, it being
possible to place a machine anywhere and to face it in any di-
rection.

(4) With motors of wide speed-range, cone pulleys and inter-
changeable gear-wheels become, to a large extent, unnecessary.

(5) The ease and quickness of speed adjustment not only
saves the time of operatives in the shops, but by encouraging a
greater care as to the proper speed to be used, insures a more per-
fect product. This is one of the greatest advantages. See
Fig. 80.

(6) Side-walls and roof-beams may be of lighter construc-
tion where shafting does not have to be supported.

(7) In cases of shut-downs, part of a plant or even isolated
and widely separated machines may be operated without the loss
of power incident to lines of shafting and pulleys.

(8) Individual motors draw power in close proportion to the
work they are doing.

As to whether original cost outweighs these advantages is a
matter that must be decided for each special case.

(f) Traction Motors.

In the operation of series motors for traction purposes, it is the
custom to use two or four machines to a car, and to make

THE MOTOR AND THE DYNAMO

the one machine or pair serve as rheostat to the other machine
or pair at the time of starting. When running at full speed, the

/Vo./

//• MINUTES

CUTTING SPEEDS AND TIME REQUIRED TO FACE A 72-INCH CAST IRON DISK
USING THREE STEPS ON THE CONE PULLEY.

! NO. 2

CUTTING SPCCD IN fr PCR Mi»
§ 3 S §

S\^

JO ZO 3O 40 50
7/MC /N MlNUTELS

CUTTING SPEEDS AND TIME REQUIRED TO FACE A 72-INCH CAST IRON DISK
WITH LATHE DRIVEN BY MOTOR WITH FIELD CONTROL.

Fig. 80. By courtesy of the G. E. Co.

machines are operated in parallel. Besides, a rheostat in series with
each machine provides the intermediate steps. The transition from
series to parallel connection is an operation of some degree of

THE D. C. MOTOR

complication. Two types of series-parallel hand controllers are in
most general use. Type K shunts and short-circuits one of the
motors when changing from series to parallel connection. Type
L controller opens the power circuit in making the change. The
series of steps in the first type is illustrated in Fig. Si. It will

#3 *4 Motor Motor

4 r-J tRjuTjiriJiriJ — °-^ — CK/W

TJ LnArulnjUL — ow — ow-
r-J LnAjiruinjb — OWY— ov\^<

-CKAA G

K>V^

dllmnJlTLJll — ow^

CONNECTIONS FOR SMALL CONTROLLERS
Fig. 81.

be observed that not every point is a running position. The rheo-
stat is not heavy enough to stand the operating current for any
length of time, and some points are passed over without being
indicated either in the motion or by marks on the top of the
controller box.

These controllers and the motors operate on voltages rang-

THE: MOTOR AND THE; DYNAMO

ing from 500 to 600 and the horse-power of the motors ranges
from 25 to 50. Above this size the multiple unit system of con-
trol is preferred. This system, used in electric trains, consists
of a master controller drawing but a small current and operated
in any car of the train and the larger motor controllers carried

T-nWi^^

CONNECTIONS FOR LARGE CONTROLLERS
Fig. 82.

with the resistances under the car and operated either by sol-
enoid coils or by compressed air, in unison with the movements
of the master controller. In changing from series to parallel
connection, the Sprague General Electric automatic control sys-
tem provides means of keeping both motors in operation and pre-
serving their torque throughout the change. (The same is true

THE: D. c. MOTOR

of the new larger type K controllers. See Fig. 82). Further-
more it sets a maximum limit relay to the rate of motion of the
controllers and so to the acceleration of the train. For fuller
acounts of these interesting controlling devices, the reader is
referred to works on electrical railway engineering, such as
Sheldon and Haussman Electrical Railways, published by Van-
Nostrand Co., 1911.

(g) The Motor-Dynamo.

This is the proper point at which to introduce the motor-dyna-

Fig. 83.— Motor-dynamo. G. E. Co.

mo, a machine having two distinct armature windings on the
same core or separate cores with a commutator at either end. It
may be used to step direct-current voltages up or down by a
given ratio, according to the relative number of inductors in the
two windings. The chief use of this instrument is as a balancer
in the Edison three-wire system. In such case the two windings
and voltages are alike. The modern type of this machine is
double in field and armature. (Fig. 83.)

THE MOTOR AND THE) DYNAMO

The balancer is employed when it is desired to run a three-
wire system from a single 22O-volt generator. Fig. 84 will make
the operation clear. As long as the system is perfectly balanced,

Fig. 84. — Three-wire S3rstem with balancer.

the balancer has nothing to do. But in the case in the figure,
the return of 5 amperes on the middle wire divides, about 2^2
amperes operating the balancer as a motor by means of one of
the armature windings and causing the other winding to gene-
rate the extra 2.y2 amperes required in the positive wire of the
circuit.

(h) Losses of Power in Generators and Motors.

In direct-current machines the losses are usually divided as in
the following table.

c (a) Watts lost in shunt field, I/R/.
Copper losses ] (b) Watts lost in series field, I/R,.

( (c) Watts lost in armature, Ia2R«.

f (d) Eddy current losses in armature iron
and pole-faces, varying approximate-
ly as the square of the speed.

I (e) Hysteresis losses in armature core,
Stray power losses <; . , . _., fi

varying as speed and as B1'6.

I (f) Bearing friction, brush friction and
windage varying approximately as
the speed.

A reference to page n, Chapter II, is all that is necessary to
make the copper losses clear. They vary with the square of the
current and hence depend upon the load. Eddy-currents occur
whenever solid masses of conducting material move rapidly

THE D. C. MOTOR 99

through an un-uniform magnetic field. The armature-core,
though laminated, is not wholly free from eddy-currents. Again,
the flux in the air-gap between field and armature is really of the
shape shown in Fig. 85.

The shifting of these tufts over the pole-face engenders in it
an e. m. f. and hence electric currents in the form of little
whirls. A similar thing occurs in the armature conductors them-
selves, especially if they have considerable superficial area.
Since e. m. f. and hence current varies as rate of cutting the
flux, and watts vary as the square of the current, these losses
vary as the square of the speed.

Hysteresis results from the reversals of magnetism in the

Fig. 85.

armature core. Once in every revolution of a bi-polar machine,
the armature iron goes through a complete magnetic cycle. The
watts lost depend upon the degree of saturation and the fre-
quency. The load carried by the machine has little influence
on the eddy current and hysteresis losses, the only effect of
the armature current in this direction being its reaction on
the field flux.

With this exception and the fact that during operation the
tension on the belt may increase the bearing friction, all the
stray power losses of the dynamo machine are the same when
the machine is running free as when it is loaded, provided the
field excitation and the speed are the same as when under load.
The stray power test of efficiency consists in determining the

100

THE: MOTOR AND THE: DYNAMO

dynamo losses, when the machine is thus running light. Then
for generators

~ . output

Per cent, efficiency = - X 100.

output -}- losses

and for motors

„ . imput — losses

Per cent, efficiency = - — : — - X 100.

input

In any machine the losses are the same, whether operating as
a generator or as a motor.

The citation of an actual case will make this clear. Connec-
tions for the test are shown in Fig. 86. Suppose a certain motor,
when loaded and operating on a circuit of 115 volts, to draw 31
amperes total current, the speed being 1,200 revolutions per

Fieldl RKeo^tat

Fig. 86. — Connections for stray-power test.

minute. If we know the field current at this time, say I ampere,
the working conditions of field and speed can be readily repro-
duced and the stray power losses determined as follows : Operate
the motor free from load, and by means of the field rheostat and
ammeter reproduce the field current of i ampere as closely as
possible. Next by means of a rheostat in the armature circuit
cut down the speed to the load speed value, namely, 1,200 revolu-
tions per minute, and read the amperes furnished to the armature
and the voltage between the brushes. Suppose these to be 2
amperes and 75 volts. The watts furnished to the armature are
then =. 2 X 75 = 150. Of these there are consumed in the arma-
ture resistance I«2Ra •= 22 X 0.5 = 2 watts, leaving 148 watts
This is known as stray power, because together with the I/R/ of

THE D. C. MOTOR

IOI

Fig. 87.— Mill type motor. G. E. Co.

Fig. 88.— Motor-operated crane. Crocker-Wheeler Co.

IO2

THE) MOTOR AND THE DYNAMO

the field it represents the power used to drive the machine at load
speed doing no work whatsoever but to overcome the opposing
forces of the machine itself. The total losses of the machine,
then, when loaded to the extent named, are this stray power loss

Fig. 89.— Motor-operated press.

plus the copper losses of field I/R/- and armature

latter are, namely, I/E/-, or i X

la. These
115 = 115 watts in the field

and 3<D2 X 0.5 = 450 watts in the armature. The total loss under
this condition of load is therefore 115 + 450 + 148 = 713 watts.

THE D. C. MOTOR 103

Now the input was IE =31 X 115 = 3.565 watts. The output
must therefore be input minus losses, or 3,565 — 713 = 2,852
watts, and the efficiency of the motor for this load and field

excitation must be—1— — - X 100 — 80 (per cent.).

The stray power losses determined in this way on a shunt
motor are fairly constant throughout a considerable range of
speeds and loads. The test may also be made on a series or a

Fig. 90. -Motor-operated lathe. Reliance Electric & Engineering Co.

compound motor, but rheostats of large carrying capacity, such as
banks of incandescent lamps or a water-barrel rheostat, must be
used. In testing a shunt dynamo machine, however, this method
is most convenient, since it draws but little power even for very
large machines. The readings of voltage, field current, load cur-
rent, and speed, must previously have been taken under conditions
of actual operation. This applies equally to testing the generator
and the motor.

The sitray-power method of obtaining efficiency gives re-
sults a little too high, owing to the fact that all defects in-
8

IO4

THE MOTOR AND THE DYNAMO

cident to full load current in the armature are lacking. The
error is however slight, and the results are likely to be more
accurate than might be determined by a clumsy or imperfect
friction brake. In cases of too great discrepancy because of
small load current, a machine may be tested by what is known as

Fig. 91.— Motor-operated milling machine. Reliance Electric & Engineering Co.

the pumping-back test. In this there are two machines exactly
alike coupled in series, one furnishing current to the other. Pre-
serving the load current in this way through the armatures, the
test proceeds similarly to the stray power test.
Figs. 87, 88, 89, 90 and 91 show a number of motor applications.

CHAPTER VI.

THE ALTERNATING CURRENT AND ITS MEASUREMENT.

(a) The A. C. Wave.

An alternating current is one which periodically reverses its
direction of flow. The alternating currents of commerce are
restricted to a certain number of reversals per second and
approximate a particular ideal wave-shape, known as a sinusoidal
curve. The following will make this clear:

The Sinusoidal Curve. — Consider the point P (Fig. 92) to be
moving uniformly around the circumference of a circle, or along
the path a b c d a. The projection of this motion on a vertical

v _y

Fig. 92. —The sinusoidal curve.

diameter through o becomes the motion o b d o. This latter is
known as simple harmonic motion, and the circle corresponding
is called the circle of reference. As the radius-vector joining
o and P sweeps around the circle, the angular displacement
(denoted by 6) of P from a passes through all values from
zero to 360°. The corresponding linear displacement from o
of the projection of P on the vertical diameter is equal at any
instant to the radius (r) X sin 0, and is known as a harmonically
varying quantity. If we draw a horizontal line ax divided to
represent degrees, and from this up and down lay off the values
of the corresponding linear displacements and join these points,
we shall have a so-called sinusoidal curve, or curve of sines;
viz., a b' c' d' x. A curve of cosines would have the same shape,
and would differ only in position, being 90° removed along the

io6

THE MOTOR AND THE DYNAMO

axis ax. Cos 0 — sin (0 -\- 90). The dotted line represents
the curve of cosines.

In order to explain why the shape of the alternating-current
wave approximates to such a curve, it is necessary to show that
the rate of change of a harmonically varying quantity, like the
sine of the varying angle 6, is another harmonically varying quan-
tity, such as the cos 0. This is expressed at once by the differ-
ds'm 0

ential calcus as

= cos 0. It can be observed also in

the curves. The increase in the sine values is greatest at a and
gradually becomes less and less till at b' variation is zero. This
variation is expressed by the first quarter of the declining dotted
curve, which crosses the axis at the 90° point. From b' to c' the
sine value decreases, at first slowly and finally with greatest
rapidity at c' . This rate of change is expressed by the second

Fig. 93-

quarter of the dotted curve, whose greatest negative value is
opposite c'. And so on.

Now let the horiontal lines in Fig. 93 represent a uniform
magnetic field of a two-pole generator, and let P be the end
view of an inductor moving around with the rotating armature.
The rate of cutting these lines of force is the e. m. f. generated
between the terminals of the inductor in absolute units (see p. 5,
Chap. II).

From a comparison of this figure with Fig. 92, it will be
seen that this rate of cutting is the rate of change of the sine
of the varying angle 0. This is, as has been shown, a harmon-
ically varying quantity. Hence the generated e. m. f., which in

ALTERNATING CURRENT AND ITS MEASUREMENT IO7

the absence of a commentator is the alternating-current e. m. f .',
can be represented by the curve of cosines, — or equally well, as
to shape, by the curve of sines.

In fact, if a be the starting point as in Fig 93, we start with
the maximum value of the e. m. f. generated. A more appropriate
point from which to measure the angles of rotation would be
where the e. m. f. is at its lowest value or zero, the lines
of force being cut with least rapidity at this point. The curve
would thus be removed 90 degrees in advance of the cosine
curve, and would be the true sine curve. If the maximum value
attained by the e. m. f. during the cycle be expressed by the
radius, or by sin 90 degrees, then the value of e. m. f . at any
moment of time t, dating from the passage of this origin by the
coil would be the maximum value of the e. m. f . X sin 6. De-
noting angular velocity by w, any angle 0 so measeared would
be w/.

Hence <?instan. = Emax sin w/.

This is made more clear by the so-called clock diagram. In
Fig. 94 let the length of radius vector o E represent Emax.

Fig. 94. — Clock diagram.

The current may be similarly expressed. Because, however,
of self-induction, which is almost always present in alternating-
current circuits, the current curve seldom coincides with the
e. m. f . curve, as will be explained later, and

Install. == Imax sin ((•)/ <&)

where 3> is the angle by which the current fmstan. lags behind
the voltage. A lagging current would be indicated by Fig. 95,
the e. m. f . reaching its maximum value while the current is still

THE; MOTOR AND THE DYNAMO

on the increase. The angle $ expresses the difference in phase
between the two.

The number of cycles ( <s* ) per second is known as the fre-
quency. There are twice as many alternations as there are cycles.
In a two-pole generator, there is one cycle per revolution. The

FiK- 95-— Lagging current.

frequency may therefore be found for any machine by multiply-
ing revolutions per second by the number of pairs of field-poles.

(b) Mean, Average and Effective Values.

An alternating current of any definite number of amperes
means a current that will have the same heating effect as that
number of direct-current amperes. The hot-wire ammeter was
one of the first forms of meter used for measuring alternating
currents. The formula for the calories (H) developed by any
current is

H = 0.24 PRf.

Thus it comes about that the effective amperes alternating-
current are not the maximum amperes expressed by the peak of
the wave, nor the mean between this maximum and zero, nor
even the average value of all the instantaneous amperes of a
complete cycle, but rather the square root of the average square
of the instantaneous amperes.

In the direct-current ammeter with fixed permanent magnet
and movable coil, the usual type, the pointer attached to the
coil moves across a scale of even divisions. Such an instru-
ment would not register alternating-current, except by a possible
trembling motion of the pointer. In the alternating-current
instrument, the magnet is replaced by a coil, the movable coil
turning in the flux set up by this fixed coil. As the current al-

ALTERNATING CURRENT AND ITS MEASUREMENT

I09

ternates simultaneously in the two coils, the deflection is in one
direction only. But this deflection is now necessarily propor-
tional to the square of the current, and the scale divisions are
uneven. The pointer does not oscillate, but because of the in-
ertia of the moving element it takes up a definite position. In this
instrument too, therefore, as well as in the hot-wire ammeter,
the deflection is that caused by the average square of all the
instantaneous current values of the complete cycle and the am-
peres marked on this scale are the square root of this mean or
average square. Thus it is that this value rather than any other
value comes to be regarded as the direct-current equivalent or
the effective alternating-current amperes. The same is true of
alternating-current volts.

Now the average value of the sines for a complete cycle of
360 degrees equals the average value of the cosines, and also

Average sin2 to/ = average cos2 to/.
But sin2 to/ -}- cos2 to/ = i for all values of to/.
Hence average sin2 to/ == */£,and ^/average sin2 ut = ily'^T.

Since ^instan. = Emax sin to/,

average <?2instan. == E2max X average sin2 to/ = tf E2max,

and Ecffective — 1/av. <?'2instau. —

V 2

= 0.707 K,

The same may be deduced practically from the following table :

Angle

Sine

Sine squared

o.ooo

o.oo

30
60
90
120

150

0.500
0.866

I.OOO

0.866
0.500

0.25
0-75

1. 00

0.75
0.25

6)3.732

6)3.00

Average sin = 0.622

Av. sin2 (at = 0.5
1 '05" = 0.707

IIO THE MOTOR AND THE DYNAMO

Because of the magnetic flux which surrounds a current-bear-
ing wire, any change of current is accompanied by a change of
flux. By Lenz's law, this change of flux due to change of cur-
rent in the wire tends to set up an e. m. f. in a direction such
as to oppose the change of current. Thus an electric current
has a property very similar to the inertia of a moving mass.
Like inertia, this is a property of the type, shape, and dimensions
of the circuit and is independent of the current in it. A helix
has more self-induction than a straight wire, and a helix con-
taining an iron core has more than one without.

Inductance (L) is measured in henries. A henry is such an
inductance as will cause a counter e. m. f . of one volt, when the
current changes at the rate of one ampere per second. This
c. e. m. f . is of the nature of an ohmic resistance, is measured in
ohms and is called reactance. Since angular speed per second in
the cycle is denoted by to, or iirf, the value of this reactance is
27T/L, where/ is the frequency.

The calculus expresses this as follows: The rate of change
of current is — ; and since i (instantaneous) — Imax sin W,

— — — wlmax cos (at = wlmax sin (W -}- 90°). This is counter

e. m. f., and the effective value thereof is wleff (or 27r/~Ieff) for
each henry of the circuit and is 90 degrees removed from the
current causing it. This being a c. e. m. f., the effective volts
applied to overcome it and cause the current to flow must be
1 80 degrees removed in phase, or 90 degrees from the current
on the opposite side. By clock diagram we have Fig. 96. IR
is plotted in the direction of the current, they being both in the
same phase. 2?r/LI is plotted 90 degrees in advance of the cur-
rent, being the impressed volts which at this frequency f is re-
quired to force current I through the circuit of L henries in-
ductance (that is, through 2?r/L ohms reactance). The resultant
of these two e. m. f.'s gives the effective e. m. f. for this circuit,

AI/TERNATING CURRENT AND ITS MEASUREMENT

III

namely OE, whose direction shows the phase relation between
current and voltage in this case.

All this can be made clear diagrammatically as follows : In
Fig. 97 the current curve abed changes most rapidly at a and c,

Fig. 96.

hence the c. e. m. f . curve is greatest at these two points, being
negative where the current is positive, and is represented by the
curve cfgh. To oppose this c. e. m. f., the impressed volts which
cause the current to flow must be represented by the cruve kflh,
which precedes the current curve abed by 90 degrees.

Fig. 97-

The vector sum of resistance and reactance is called imped-
ance. The reciprocal of impedance is called admittance, sim-
ilarly as the reciprocal of resistance in direct-current is called
conductance. Impedance, like reactance is measured in ohms.

112 THE MOTOR AND THE) DYNAMO

(d) Capacity in Circuit.

When an alternating e. m. f. is applied to the terminals of a
condenser, the latter is charged and discharged in rapid succes-
sion, each plate receiving alternately a -|- and -- charge. The
effect is the same as if the alternating-current went through the
condenser, which offers a resistance effect to the flow of cur-
rent,— also a form of reactance. But whereas inductance causes
a lagging current, a condenser, or capacity, in the circuit causes
the current to precede or lead the e. m. f. in phase.

The calculus explains this as follows : If K is capacity in
farads, E pressure in volts, and Q quantity of electricity in
coulombs or ampere-seconds, then

Q = KE

It must be remembered that a farad is that capacity which
will receive a charging current of one ampere when the e. m. f.
is changing at the rate of one volt per second.

The condenser current 4 at each instant is proportional to the
rate of change of pressure, or

But ^instan. == Emax Sin

de_
dt

and —'- = (O Emax COS

Hence -~ = Emax cos to/ = Emax sin (o>/ -J- 90°) = e^ where
o>K

Ck is the instantaneous pressure in phase with the condenser cur-
rent. Hence the effective value of Ik divided by o>K (or by 27T/K)
is the value of the effective e. m. f . causing this current Ik and
is 90° behind I in phase.

In Fig. 97 let efgh represent the e. m. f. wave. At e the
e. m. f. is changing least rapidly, and so the current flowing into
the condenser is least, giving a as the current point. At /
the e. m. f . is changing most rapidly, hence current value is high-
est, or at b. From / to g the e. m. f . is still increasing, but less
rapidly, hence the current, although still positive, decreases, and
so on.

ALTERNATING CURRENT AND ITS MEASUREMENT

In the vector diagram,

27T/K

would be plotted therefore

1 80 degrees removed from 2?r/LI, or in the opposite direction.
Hence in the vector diagram Fig. 96, it should be laid off down-
ward. The complete effect of resistance, inductance and capacity
in a circuit would be represented vectorially as in Fig. 98.

, the reactance due to capacity and 27T/L,, the react-

ance due to self-induction, must be added algebraically. In
this case 27T/L is the larger. The vector sum of these reactances

-ir

(X) and the resistance (R) of the circuit is represented by
the hypothenuse of the triangle and is the impedance (Z) of the

circuit. Hence Z =

It is seldom possible to have resistance in an alternating-cur-
rent circuit without inductance and vice-versa. Let there be, how-
ever, an ideal circuit composed of pure resistance, pure induc-
tance and pure capacity, connected in series, and let the drop over
each part of the circuit be obtained separately by a voltmeter,
the current in the circuit being maintained constant, then the
vector-sum of the resistance drop and reactance drop will be the
impedance crop, or difference of potential across the circuit as in
Fig. 99.

114 THE MOTOR AND THE DYNAMO

(e) Power in A. C. Circuits.

From the fact that the voltage and current are seldom in
phase in alternating-current circuits, it is at once apparent that
the volt amperes is usually larger than the watts. The ratio
of these two quantities is known as the power factor, and is the
cosine of the angle (f> on the vector diagrams Figs. 96 and 99.

Treating Fig. 99 as a clock diagram, counter-clockwise rota-
tion being regarded as positive, let the volts be plotecl in the
direction OE. The current will then be in the direction OI,
lagging behind the volts by the angle <f>, due to impedance. This
direction will correspond to that of the resistance drop, since

T- 99-

pure resistance causes no change of phase. The component of
OE in the direction OI is then OE cos <£ and
watts

volts X amps.

— cos <£ = power factor.

We thus see that the impressed volts in an alternating-current
circuit are made up of two components, one overcoming the resist-
ance and the other the reactance. It is sometimes of advantage
to interchange OE and OI on the diagram, considering clockwise
rotation as positive. Then one may speak of two components of
current, the power component, which is in the direction of IR,
and the wattless component at right angles to it.

ALTERNATING CURRENT AND ITS MEASUREMENT 115

The formula may be deduced mathematically, as follows :
e = E sin <ot
i = I sin (a>/ — <£)
ei = El sin w/ sin (W — <£)
sin (w/ — <£) = sin <*>/ cos <£ — cos w/ sin <£
« = El sin2 to/ cos <£ — El cos W/ sin to/ sin <£.

Average « = El cos <£ av. sin2 to/ — El sin <£ av. (cos to/ sin to/) .

But av. sin2 o>/ = ^ and av. sin to/ cos to/ = o

hence, av. « = watts = - - cos <£

which is the same as - cos <£ or Eeff Ieff cos <#>.

1/2 1/2

This cos <£ is again the power factor, or that quantity by
which the effective volt amperes must be multiplied to obtain
effective watts.

Since watts is a rate of working, it may be represented by an
area formed by the products of volts by amperes at each succes-
sive instant of time. Those areas which lie above the line,
being due to the product of quantities of like sign, are posi-
tive and denote power furnished by the circuit. Those which
lie below the line are negative and represent power withdrawn
by or used up in the circuit. See Figs. 100, 101 and 102.

(f) Alternating Current Measuring Instruments.

It will be remembered that the most common type of direct-
current ammeter or voltmeter consists essentially of a movable
coil operating in the field created by a permanent magnet. Ob-
viously with an alternating current in the coil no steady position
of the needle could be maintained. In fact, unless the alterna-
tions were very slow or the movable parts very free from in-
ertia, the needle would only tremble slightly back and forth or
would refuse to move at all. This difficulty is avoided by elimi-
nating iron from the instrument and allowing a stationary coil
bearing the current to furnish an alternating, that is, continually
reversing, magnetic field. The movable coil is supplied from the
same source and the current in it alternates at the same rate and

MOTOR AND THE DYNAMO

TTS

Fig. 100. — Power in a non-inductive circuit.

Fig. ioi.— Current lagging 90°. Algebraic sum = zero. So-called wattless current.

ALTERNATING CURRENT AND ITS MEASUREMENT

117

in synchronism with the magnetic field. This creates a uni-
directional torque on the movable coil.

In the case of the wattmeter as well there are two coils,
one movable and the other stationary. The one, however, is in
series with the circuit, is of low resistance, and bears the current
(amperes). The other is of high resistance and voltmeter-like
is tapped across the circuit. The combined effect, therefore, is
at each instant proportioned to the product of the volts by the

Fig. 102.— Usual lagging current. Power- factor less than i.

amperes. With such instruments there is necessarily a power-
factor of the wattmeter itself which varies with the character
of the circuit. It is, however, usually small.

An instrument having an extremely light movable part has
in recent years been perfected, known as the oscillograph. In
this instrument, by means of a mirror oscillating with the cur-
rent, a photographic record may be taken of the current curve,
and simultaneously on the same film by means of other mirrors,
each on its own motive device, the e. m. f . wave or any other de-

THE MOTOR AND THE) DYNAMO

sired may be photographed. Fig. 103 is a reproduction of such a
photograph.

(g) Voltage in A. C. Circuits in Series.

It is possible to have a non-inductive circuit, that is a pure
resistance, but the reverse, a pure inductance without resist-

Fig. 103.— Sinusoidal curves. Courtesy of G. F,. Co.

ance is of course impossible. The theoretical diagrams, there-
fore, thus far given have to be modified in practice. Let the
circuit represented in Fig. 104 consist in part of a pure re-
sistance and in part of an impure inductive circuit or an imped-

Fig. 104;— Series circuit.

ance, and let the voltmeters show the drops across the differ-
ent parts of the circuit when current is flowing. If then the
three voltmeter readings be laid off in vector diagram, Fig. 105,
it will be found that the angle at c is not a right angle.

ALTERNATING CURRENT AND ITS MEASUREMENT

119

The distance of cd ought theoretically to represent the resist-
ance drop of the inductive circuit and is indeed of the nature
of a resistance drop, but is considerably greater when obtained

by alternating-current than by direct-current, owing to hysteresis
and eddy currents in the inductive apparatus.

( h) Current in A. C. Circuits in Parallel.

When alternating circuits are arranged parallel, the total volt-
age must of course be the same as that over each part.

When alternating-current circuits are in parallel the total cur-
rent is the geometric sum of the current in the branches. Fig.

Tl z
1 ^0000

Fig. 106. — Parallel circuit.

106 represents two such circuits, the one containing pure re-
sistance R, the other an impedance Z.

Let the horizontal line in Fig. 107 represent the current Ilt
9

120

THE MOTOR AND THE DYNAMO

shown by ammeter Ax, the current and e. m. f. of this circuit be-
ing in phase. The phase angle between !,_ and I2 can be found

Voltage direction.
1,

total

by the formula

Fig. 107.

= cos <f>. Complete the parallelogram and

the diagonal will be the value of Itotai read by ammeter At.

(i) Two-Phase and Three-Phase .

It was seen on page 14 that by means of rings tapped on to
the armature winding of a direct-current generator at points
separated by the polar span, an alternating e. m. f. could be ob-

Ph.l.

Fig. 108.— Two phase.

tained. This would be single phase alternating-current. If now
two more rings were similarly tapped onto the armature wind-
ing at points in quadrature to these, or at a phase difference of
90 degrees, the two-phase current or e. m. f. lead off on the four
wires would be as represented in Fig. 109.

ALTERNATING CURRENT AND ITS MEASUREMENT

121

The same result could be obtained by means of two single
phase two-pole generators whose shafts are coupled in a position
represented by Fig. 108. The relative position of the poles in

Z7<>

1360

Fig. 109.— Two phase.

quadrature is to indicate the relation in space of the two e. m. f.
curves. The clock diagram corresponding to Fig. 109 as to volt-
age and with a lagging current is Fig. 1 10. When the currents in
the two phases are equal and <f> is the same for each, the sys-
tem is said to be balanced. In such a case as this, three wires
may be used instead of four. The current in the joint or middle
wire is then 1.41 times that in either outside wire, that being

re*.

pK.Z

Fig. no. — Two-phase clack diagram.

the ratio of either side of the square to the diagonal. Hence in
a two-phase alternator the current capacity of the armature wind-
ing is 41 per cent, more than if the same machine were wound

122

THE; MOTOR AND THE: DYNAMO

for one phase, heating and energy losses being the same in the
two cases.

If the direct-current armature and commutator above referred

Fig. i ii. —Three-phase A. Fig. 112.— Three-phase Y.

to were tapped at three points 120° apart, a three-phase current
and voltage could be obtained. The figures for three-phase cor-
responding to Fig. 108 and Fig. 109 are Figs, in, 112 and 113.
In Fig. in represents what is known as the delta (A) method of

Fig. 113. — Three-phase.

connecting and Fig. 112 represents what is known as the star or
Y method of connecting the three distinct armature coils or wind-
ings, as in Figs. 114 and 115.
In the A system the voltage between any two line wires is that

ALTERNATING CURRENT AND ITS MEASUREMENT

I23

generated in the armature coil from which they spring. The
current in each line wire, however, is the vector sum of the

Fig. 114. Fig. 115.

currents in the adjacent armature coils. If the system is a
balanced one, the line current in each wire will be j/J times the
current in one armature winding, as appears from Fig. 116.

Fig. 116.— A-current.

In the Y system, the current in any line wire is the same as
that in the armature winding from which it leads. The voltage,
on the other hand, between any two line wires is V3 times the

Fig. 117.— Y-voltage.

voltage generated in one armature winding, as appears from
Fig. 117.

CHAPTER VIII.

ALTERNATING CURRENT MACHINERY.

(a) A. C. Generators.

In polyphase current alternators, except in very small machines,
the continuous winding corresponding to the tapped direct-current
armature above referred to is not used. Machines for the gen-

Fig. 118.— G. E. Co. alternator armature-(stator) winding.

eration of the commercial alternating-current employ instead
separate windings as suggested by Figs. 108, in and 112. It is
furthermore usual to place these armature windings on the inner
side of the stationary frame of the machine and to employ a ro-

ALTERNATING CURRENT MACHINERY

125

tating field. The small exciting direct-current is conveyed to this
field by means of rings. The armature current then is drawn
direct from the windings, without any brush contacts. See Figs.
118, 119, 120 and 121.

Fig. 119. — G. E. Co. alternator, showing core laminations, frame and field poles.

The exciting current for the fields of large generators is usually
derived from a separate small generator. In power plants one
such field generator may be used to supply several machines.
For isolated alternating-current generators the field exciter is
sometimes direct coupled to the shaft of the alternator. In
smaller machines with stationary field, a commutator may be
provided for field excitation.

(b) Voltage Regulation of the Alternator.

Alternators, like direct-current generators, have the magnetiza-
tion curve similarly determined, and also the external character-
istic curve. The latter depends greatly on the character of the
load, whether it be inductive or non-inductive.

126

THE MOTOR AND THE DYNAMO

The voltage regulation of an alternator, as also of a direct-
current generator, is technically expressed by the equation

t . voltage at no load — full load volts

regulation = - — —

full load volts

and gives the per cent, rise in voltage resulting from a sudden
reduction of the load to zero.

In very large machines, such as those in central power stations,

Fig. 120.— G. K. Co. three-phase stator-winding of alternator.

this ratio is difficult to determine by direct readings, it being
usually impossible to load such machines to their full capacity,
because of the difficulty in supplying resistance suitable for
receiving the full-load current. The regulation has therefore to

ALTERNATING CURRENT MACHINERY

127

be arrived at by an indirect experimental test. The problem
will be made clear by the following consideration.

The armature circuit of the alternator, like any other circuit,
contains resistance and inductance. The drop over each of these,

Fig. i2i.— Portion of stationary-armature winding of a three-phase alternator.

under condition of any given load, is represented in Fig. 122 by
IR* and IX« respectively, the load being a non-inductive one.

Figs 123 and 124 show two different cases with inductive load.
The first case is such that the impedance of the armature circuit
increases the phase angle between generated volts and current.
In the second case the armature impedance decreases this angle.

128

THE MOTOR AND THE DYNAMO

Ra can be determined by the methods used in direct-current
generators. One method of arriving at the reactance of the
armature circuit is the following: With the field circuit of the
alternator open, the armature terminals are short-circuited
through an ammeter and the field is cautiously excited until the
ammeter reads full-load current. The voltage now generated is

LOAD VOLTS

I R

Fig. 122.— Voltage regulation with resistance load.

all used up in sending this current through the armature circuit.
By opening the short-circuiting switch and keeping the speed and
field excitation constant, the voltage generated under these con-
ditions may be read on a voltmeter.

This voltage includes the drop at full load due to armature

CURRENT DIRECTION
Fig. 123. — Voltage regulation with inductive load.

resistance and reactance and also includes the effect of armature
reaction on the field flux. Because the first of these three factors
is very small and because the third has an effect on the power
factor of the machine similar to reactance, the voltage thus

ALTERNATING CURRENT MACHINERY

129

obtained is called the drop due to synchronous reactance and
may be considered the IXrt of' the preceding^diagrams.

The method here given, known as the electro-motive-force
method of determining voltage regulation, is not an accurate one,
owing to the fact that the field being necessarily weak, the arma-
ture reaction is excessive. In a polyphase machine, however, the
result is likely to be nearer the true value than in a single-phase

J-B

CURRENT DIRECTION

Fig. 124.

generator. For a more elaborate treatise on this subject than
is allowed by the scope of this book the reader is referred to
Thomalen's "Electrical Engineering."

The inductor alternator differs from all other types of electric
generator in that the windings, both field and armature, are
stationary, and the iron alone revolves. The armature is wound
on the frame, as in other alternators, and the field cores consti-

130

THE MOTOR AND THE) DYNAMO

tute the rotating part. This machine, therefore, is very rugged
in construction. Its alternating-current wave is almost a pure
sine curve, and in power-factor and efficiency it compares favor-
ably with the ordinary type of alternating-current generator. It

Fig. 125. — Inductor alternator with vertically split armature.

is especially well adapted for a widely varying load at low voltage,
as is demanded for electric welding, etc. See Fig. 125.

(d) The Compounding of Alternators.

A series winding is sometimes added to the field of an alterna-
tor for the purpose of maintaining a constant or an increasing
voltage with increase of load. Formerly, one method largely em-
ployed consisted in shunting off a portion of the main armature
current and passing it through a rectifier. This was simply a
commutator having as many segments as there were field poles
and mounted on the shaft of the machine. The brushes took off

ALTERNATING CURRENT MACHINERY 13!

a pulsating current which supplied the series field and varied with
the main armature current.

The modern method is to vary automatically the voltage im-
pressed on the field of the alternator by its direct-current exciter.
The automatic device controlling this operation is termed the
Tirrell regulator, and the connections are shown in Fig. 126.
Briefly 'its operation is as follows:

When the exciter voltage falls too low, the direct-current con-
trol-magnet on the left is weakened. When the alternating-cur-
rent generator voltage falls, the solenoid magnet to the right is

Afa/n Contacts

AC fie/d >4 C Generator
Rheostat

ELEMENTARY DIAGRAM OF TA. FORM A REGULATOR
Fig. 126. G. E. Co.

weakened. Either or both of these operations close the main con-
tacts, according to the adjustments of the counterweight. This neu-
tralizes the relay magnet, closing the relay contacts, and so short-
circuits the exciter field rheostat and raises the exciter voltage. This
increase of voltage re-opens the main contacts. "The operation is
continued at a high rate of vibration, due to the sensitiveness of
the control-magnets, and maintains not constant but a steady
exciter voltage." A compensating winding on the alternating-
current control magnet is connected to a current transformer in
the main line and causes an increase of voltage with increase of
load, thus taking care of the line-drop.

132

THE MOTOR AND THE DYNAMO

(e) The Synchronous Motor.

In treating of the direct-current motor, it was shown that rota-
tion is produced by the action of the field flux on the current-
bearing armature conductors. In order that this thrust may pro-
duce continuous rotation, a commutator is required to change the
direction of the current in each conductor twice in each cycle,
that is, to produce in the conductor an alternating current whose
direction changes simultaneously with the passage of the con-
ductor from pole to pole. If therefore the field continues to be

Fig. 127. — A large alternator in process of construction.

excited with a direct current, and the commutator of such a motor
be replaced by rings, and an alternating current of the proper fre-
quency be fed into the rotating armature, the motor will continue
to run at a constant speed in synchronism with the alternating
current. To operate such a motor on the alternating current, it
must obviously first be brought up to speed and also into the

ALTERNATING CURRENT MACHINERY

133

correct phase relation with the given current. Should it be
slowed down so as to fall out of step with the given alternating
current source of supply, it will immediately stop.

Fig. 128 shows a device used in starting such motars. It con-

SYNCH

Fig. 128.— Connections for synchronous motor.

sists of lamps bridged across the switch between the motor and
its generator. These lamps prevent the flow of an excessive
current and serve to indicate the relative frequency and the phase

(a) (b)

relation of the two machines. Let the generator be driven by
its engine or other prime mover at a definite and constant speed,
and let the motor be started by some device, say a small direct-
current motor, whose speed can be regulated. Each machine will
now be generating an alternating-current e. m. f. If these
e. m. f.'s are opposed to each other in direction through the lamps,
it is evident that no current will flow between the machines, and

134 THE MOTOR AND THE DYNAMO

the lamps will be dark. The e. m. f /s are opposed to each other
in phase (see (a) ). If on the other hand, the e. m. f.'s are so
related as to send the current in the same direction through the
lamps, they correspond in phase as regards this circuit (see (b) ),
and the result will be an increased voltage across the lamps caus-
ing them to glow. If either machine has a different frequency
from the other, there will result alternate reinforcement and
interference, and the lamps will flicker as in (c),

The method is to regulate the speed of the motor so that the
flickering becomes very slow, and then to close the switch in the
middle of a dark period. This may be varied by having the
lamps cross-connected. The switch should then be closed in the

Fig. 129.— Vectordiagram of synchronous motor.

middle of a bright period. If now the starting device be mechan-
ically disconnected or its driving circuit opened, the synchronous
motor will continue to operate on the alternating-current fed into
it. This will be clearer if the e. m. f . generated by the motor in
starting be considered its natural counter e. m. f ., as in the direct-
current motor, which is opposed in direction to the impressed
e. m. f. of the driving circuit.

It will be remembered that the direct-current motor draws cur-
rent in proportion to the load put upon it, because of the retarda-
tion in speed caused by the load. In the synchronous motor,
being a constant speed machine, this cannot be the case. The
operation of this motor is to be explained by the vector diagram,
Fig. 129.

ALTERNATING CURRENT MACHINERY 135

L,et the e. m. f. of the generator E. be considered positive and
so plotted toward the right from the origin O. The counter
e. m. f the motor E,* will then extend toward the left. But
although at the instant of connection to the circuit the motor may
be in direct opposition to the generator, when its starting device
is cut off, it instantly falls somewhat behind the 180° phase; that
is, its vector will take the direction OE,«. These e. m. f.'s are
in a series circuit. Following the usual method of combining
e. m. f.'s in series by completing the parallelogram, we have the
resultant e. m. f. OE^, which sends the driving current through
the motor armature. Because of the impedance of the motor
circuit, this is a lagging current, or OI, the cos^> depending solely
on the character of the motor circuit and being therefore a
constant.

Now suppose the load on the motor to be increased. This

I
'I"

Fig. 130. — Vector diagram of synchronous motor.

tends to cause a slackening of .speed, but what really happens is
that E,« swings into a new relation with EAO namely Ew', giving
a new resultant E/, and current I'. The impedance of the cir-
cuit being essentially constant, I increases with the increase in Er.
It will be remembered that the speed of a D. C. shunt motor
may be increased by resistance in the field circuit. A change in
the field current of the synchronous motor serves only to shift the
phase relation between Ee and E,M. In Fig. 1 30 the vectors OE^ and
OI correspond in direction, and the power factor of the driving
current is unity. Should E,« be increased, by an increase in the
motor field current, to E,,/, then Er and I would have new posi-
10

136 THE MOTOR AND THE DYNAMO

tions, namely E/, and I'. Likewise should E,« be decreased to
E,,/', then Er and I would become E/' and I". The lower values
of K,« therefore cause the current from the generator to be a lag-
ging current and the higher values of E,« cause the synchronous
motor to have the effect of a capacity, giving the generated cur-
tent a position of lead.

For any given power input into the motor, there is one value
of motor field at which the driving current is a minimum, a
change either way causing an increase in the current for the same
amount of power delivered to the motor. This gives rise to the
so-called V curves plotted between motor current and motor
e. m. f .

A synchronous motor with over-excited field may be used to
improve the power factor of a transmission line.

An inspection of Fig. 129 will make it evident that the syn-
chronous motor is limited in the load it can carry without pulling
out of step. Ew can shift phase with respect to E^ until the
angle between E,« and motor current I is 90°. This means a
current of zero power factor, and the motor will stop when this
angular relation is reached. The stopping of the synchronous
motor is sudden, when it has pulled out of phase. The actual
operative range of the synchronous motor is over a smaller angle
than that indicated by this 90° limit, because of internal losses
in the machine. The stopping of the motor under these condi-
tions is the same as would happen in a direct-current machine,
if commutation were to occur not on an axis at right angles to
the field flux but 90° removed from this point.

(f ) The Operation of A. C. Generators in Parallel.

Instead of generator and synchronous motor, consider the two
machines just under discussion to be two alternating-current gen-
erators. Let one generator be furnishing current to a circuit.
The other generator can be brought up to speed and voltage,
synchronized and connected to the bus bars in the same manner
as the synchronous motor. When once so running and connected,
two alternators tend to remain in step. For let one of them be
supposed to drop behind, it immediately becomes a synchronous

ALTERNATING CURRENT MACHINERY 137

motor, the resultant e. m. f., E,- of Fig. 129, sending a current
through its armature. This relieves the load on its driving engine
and tends at the same time to retard the other machine, so that
the two swing again into step. This very action, however, has
proved to be a source of great trouble when the governors of
the engines are of quick action. For in that case it is found
that the heavily loaded machine is immediately supplied with more
power, the driven one with less, which interferes with their
natural tendency to fall again into step. Heavy fly-wheels also
interfere with this tendency to remain in synchronism by carry-
ing the retarded or aided machine beyond its proper position and
so leading to the phenomenon known as "hunting."

It will be recalled that when direct-current generators are
operated in parallel, the distribution of the load between the two
machines is regulated by their field rheostats, controlling their
relative e. m. f 's. This is not the case with alternators. An
increase of voltage of either generator gives rise to a shift in
the phase relation of the two machines and a useless cross-current
between them. There is therefore one position of the field rheo-
stat for each alternator operating in parallel with others such
that the sum of all the currents shall just equal the total output
of the station. A change in either direction from this position
indicates an uneconomical adjustment and a loss of power.

The means of controlling the output of the various alternators
in parallel connection is to be sought therefore in the power sup-
plied to the prime movers, as, for instance, the steam supplied
to the engines. By regulating this, the load on the various gen-
erators can be controlled.

For synchronizing generators in power stations, preparatory
to throwing a machine into service, a device with a hand and
dial known as a synchroscope is generally employed in place of
the less reliable lamps. The direction of rotation of the hand
indicates whether the machine to be connected is running too
fast or too slow. A stationary hand indicates perfect synchronism
and a hand stationary in a vertical position indicates perfect
synchronism and equality of phase. See Fig. 131.

138 THE MOTOR AND THE DYNAMO

As regards the motive power of alternating-current generators

Fig. 131.— Synchronism indicator. G. E. Co.

operating in parallel, reciprocating steam engines are still found
in many large plants. But because of the irregularities in the
period of rotation of any reciprocating engine due to the very

Fig. 132.— Crocker-Wheeler Co. a.c. generators driven by reciprocating steam engines.

nature of its construction, turbines, both steam and water, seem

ALTERNATING CURRENT MACHINERY

139

Fig. 133. — G. E. Co. 8,000 k.w. a.c. generators with vertical shaft operated by
Curtis steam turbines.

Fig. 134.— Crocker-Wheeler Co. 4,000 k.v.a. generators operated by gas engines.

140

THE; MOTOR AND THE DYNAMO

now to be preferred as prime-movers for alternators. Especially
water-power drive has been rendered very perfect to-day by the
invention of an oil-pressure governor. The oil is kept under
pressure by a pump, either controlled or directly operated by the
turbines. A change in speed of any generator and turbine results
in a change of oil-pressure. By a system of pistons and corn-

Fig. 135.— 4,000 k.w., 2,2oo-volt generators at Niagara Falls. G. E- Co.

pound levers, this change of pressure is made to operate the
guide-vanes of the water-wheels, thus controlling the direction
and amount of the water entering the wheel. This system is in
use in the new power-house on the River Rhine near Basel and is
being installed in the great power-house now under construction
at Keokuk on the Mississippi River.

(g) The Rotary Converter.

The so-called rotary converter is essentially a shunt generator
or motor with the usual commutator mounted at one end of the

ALTERNATING CURRENT MACHINERY

141

armature and with the rings tapped on according to the number
of poles and of phases. For instance, in the central distance
between two like field poles there must be two taps, one to each
ring for single phase, three for three phase and four for two
phase, in each case equally spaced. See Fig. 136.

Fig. 136. — Rotary-converter armature connection.

The machine may then be run from the direct-current end and
be made to furnish alternating-current or from the alternating-
current end and furnish direct-current. See Figs. 137, 138 and
139. The latter method is the usual one. When run from the
direct-current end, the machine is spoken of as an "inverted
rotary."

142

THE: MOTOR AND THE; DYNAMO

The chief use of the rotary converter is to be found in the
sub-stations of light and power companies. In order to minimize
expense of transmission, it is the custom to generate power in the
central station at a high alternating-current e. m. f . This can
be transported to great distances over comparatively small wires.
In order to be of commercial value, it must be stepped down to a
lower voltage, which is easily accomplished by means of the

Fig. 137.— G. E. Co.— A modern rotary converter. D. C. end.

transformer. This instrument will receive a brief notice later.
But the transformer delivers the power in the alternating-current
form. This is serviceable for both incandescent and arc lighting
and for operating certain types of motors. There are, on the
other hand, applications to which direct-current is much better
fitted than alternating-current, as, for instance, traction machinery
and motors of varying and controllable speed. Hence in the sub-
stations of traction companies the high pressure alternating cur-
rent received from the main power-house is usually stepped down

ALTERNATING CURRENT MACHINERY

143

by means of a transformer and then supplied to the alternating-
current end of a rotary converter, from which direct-current is
sent out over the feeders to different points of the system.

The behavior of the rotary converter, when operated from the
alternating-current end, is very similar to that of the synchronous
motor. If directly connected to the line, without the interven-
tion of a transformer or other inductive circuit, a variaton of

Fig. 138.— G. E. Co. rotary converter. A. C. ends.

field excitation will produce a shift of phase with reference to
the supply voltage, but will not much alter the direct-current
volts delivered. As in the synchronous motor, the amount of
field excitation also determines the phase relation between the
current and the impressed alternating-current e. m. f. In case
there is inductance in the supply circuit, as, for instance, a trans-

144

THE MOTOR AND THE DYNAMO

former, a change of field current may also vary the generated
counter e. m. f. and therefore the direct-current volts, but not
over a wide range. In the machine shown in Fig. 139, the most

Fig. 139.— G. IJ. Co. regulating-pole rotary converter.

recent type, the direct current volts are successfully regulated
over a range of 10 per cent, in either direction by means of sepa-
rate regulating poles. When operated from the direct-current

ALTERNATING CURRENT MACHINERY 145

end, a change of field current, although it may increase or
decrease the speed as in a shunt motor, has no effect on the
alternating-current volts unless the machine is loaded, and then
only a slight one.

The theoretical relationship between direct-current and alter-
nating-current volts, whether the machine is operated from either
end or is driven as a generator by some prime mover, would be
as follows: —

D. C. A. C.

Single phase 100 70.7

Three phase 100 61.2

Two phase 100 50.0

This table is on the basis of a sinusoidal e. m. f. In the actual
converter, however, this condition is never realized, and the ratio
between direct-current and alternating-current e. m. f.'s varies
considerably from these values, even at zero load.

When loaded, the armature of the rotary carries both alter-
nating-current and direct-current. In any armature conductor
the external direct-current becomes alternating through the
agency of the commutator. Thus each conductor has in it two
alternating-current components of current, the one theoret-
ically sinusoidal, the other with more abrupt rise and reversal.
And these two components are not in phase. The wave-shape of
the resulting current therefore departs altogether from the sinu-
soidal form.

From the foregoing paragraphs it will be clear that there must
be two forms of armature reaction present in the loaded machine.
The one distorts the field flux in a forward direction, the other
against the direction of rotation. The direct-current brushes are
therefore to be set on the geometrical axis of symmetry between
the poles.

When the machine is operated from the direct-current end and
an inductive load is placed on the alternating-current end, the
lagging causes a field weakening and an increase in speed which
may attain dangerous proportions.

In sub-stations where there are several rotary converters, it is
the custom to start the machines from the direct-current end,

146

THE MOTOR AND THE DYNAMO

when they can readily be brought up to speed and synchronized.
As one or more machines are always in operation, the requisite
direct-current supply is always at hand.

Rotary converters have the fault of hunting, just as alternators
when operated in parallel. In both machines this is to a great
extent obviated by inserting heavy copper conductors of various

forms, I \ J, etc., in the faces of the pole pieces, the

pole pieces themselves being laminated. A heavy steel shoe across

Fig. 140.— I^arge rotary converter in process of construction. Transformers
in the distance.

the face of the laminated pole piece has a similar effect. The
eddy-currents set up in these give steadiness of motion and pre-
vent hunting.

(h) The Transformer.

Although the transformer is neither motor nor dynamo, yet a
brief notice of this device must be included in the present volume.

ALTERNATING CURRENT MACHINERY 147

It consists of two spools, or windings, of insulated wire placed
on a core of laminated iron or surrounded by the same, and
usually immersed in oil contained in an iron casing. The func-
tion of the oil is to cool and insulate. Forms are shown in Figs.
141, 142 and 143. The alternating flux created by the one coil
cuts the other coil, establishing in it an alternating e. m. f .

The coils are known as primary and secondary, and ignoring

Fig. 141.— The transformer. G. E. Co.

losses the respective voltages are in direct ratio to the number of
turns in the coils, the currents being in the inverse ratio. The high
voltage coil is usually termed the primary, being the one to which
the power is furnished, the transformer being chiefly used to
step down the voltage of a transmission line. In power stations,
however, the instrument is sometimes used to step up the voltage.
It is the transformer which furnishes excuse for the existence
of the alternating current as a commercial form of power. Power

148

THF, MOTOR AND THE DYNAMO

at a high voltage with small current is far less expensive as
regards copper and PR loss when transmitted to considerable
distances than the same power at low voltage with large current.

Fig. 142.— Wagner lo-kw. lighting transformer element.

The transformers more than pay for themselves on such a high
voltage line, and their efficiency of operation is high, from some
94 to 98 per cent. The ratio commonly used is ten to one.
The losses in transformers are therefore small. The iron loss,

ALTERNATING CURRENT MACHINERY 149

that is, the power used for overcoming hysteresis and eddy-
currents in the core or shell, as the case may be, ranges from
0.6 per cent, to i per cent, in modern transformers and are prac-
tically constant for all loads. The copper losses, PR, of both
windings, range from i.i to 1.8 per cent.

The no load, or exciting, current is very small, owing to the

Fig. 143. — Automatic constant current transformer for series
lighting system. G. E. Co.

c. e. m. f. of self-induction in the primary circuit. The power
factor at no load is also very small, the resistance being compara-
tively low and the reactance high. The exciting current, there-
fore, lags almost 9x3° behind the primary volts. When the load
current is drawn from the secondary windings, however, it is at
the expense of the primary flux, tending to reduce it. This in

THE; MOTOR AND THE; DYNAMO

turn tends to lower the c. e. m. f . of self-induction and allows
the primary current to increase in value in proportion to the load.
The flux of the core, or shell, is in this way restored and remains
constant in value throughout all loads, and the transformer draws
power automatically from the mains, according to the demands
made upon it.

Since the capacity of the transformer depends on temperature,
and the heat developed in the coils depends on current independent
of power-factor, transformer capacity is usually expressed in
kilo-volt amperes, K. V. A., rather than in kilowatts. In sizes

100 volts

12.5 volts

Fig. 144. — The auto transformer.

over 40 K. V. A. the retaining case is usually corrugated to aid
the cooling, and in the largest sizes recourse is had to the air-
blast or to water circulation.

The auto transformer consists of a single turn on a laminated
core, and its action is much like that of the potentiometer in
direct-current service. Unlike this instrument, however, voltage
may be stepped up as well as reduced by the auto transformer.
See Fig. 144.

(i) The Induction Motor.

The induction motor receives its name from the fact that the
current in what corresponds to the armature is not drawn from
the supply mains but is induced. The armature, more properly
called the rotor, is not electrically connected with the outside
source of supply. Its current is generated by the alternating flux
from the poles of the stationary part, or stator.

In Fig. 145 the stator is wound for a two-phase four-wire cir-
cuit, having two poles to each phase. There are thus two dis-
tinct windings. Winding AC creates a flux such that the stator

ALTERNATING CURRENT MACHINERY 151

iron has a north pole at b and a south pole at d. The winding
BD on the other phase is at this time dead. As the current in
AC dies out, that in winding BD builds up (see Fig. 146), so
that the north pole is gradually shifted from b to c and the south
pole from d to a. In the second quarter of the cycle, the current
in winding CA builds up in the reverse direction, that in BD

Ph.a

Fig. 145.— The induction motor. Diagram.

dying out, thus shifting the north pole to the positions d and the
south pole to b. By continuing this analysis through the next
two quarters, it will be seen that the polarity of the stator iron is
made to rotate, in this instance once around for a complete cycle.
Were there four poles per phase, it is evident that it would
require two complete cycles to cause the polarity of the stator
ii

152 THE MOTOR AND THE DYNAMO

iron to make a complete revolution. That is, the number of

revolutions of the stator magnetism per secoud, n, is — —, — where

Yzp

f is the frequency and /> the number of poles per phase, or

•-f-

A rotor consisting of a solid cylinder of iron would experience
the drag of this flux, and if the friction were not too great, would
rotate in synchronism with it. A cylinder or other centrally
symmetrical form of any metal would also be put in motion
because of the eddy-currents set up in it by the stator flux.

The rotor of the induction motor consists of the usual laminated
iron core, either wound similarly to the armature of an alternator
or pierced near its periphery by stout copper bars connected in

Fig. 146.

The latter is known as a squirrel-cage rotor in distinction
from the wound rotor. By a reference to Fig. 145 it will be
seen that these windings or bars are not unlike the secondary
windings of a transformer, and have for the same reason an
alternating e. m. f . induced in them. It is the resulting current
on which the stator flux acts in producing rotation. And because
the windings or bars give the proper direction to this induced
current, the torque effected is much greater than could possibly
be the torque on a solid metal cylinder.

If the speed of the rotor be such that it is in exact synchronism
with the rotating stator flux, then the rotor conductors will be
fixed in their relation to this flux, and no current will be induced
in them. As a rule, however, the speed of the rotor is slightly
less than that of the stator flux, the ratio of this difference to

ALTERNATING CURRENT MACHINERY 153

the speed of the stator flux being technically known as the slip;
that is,

r.p.m. of stator — r.p.m. of rotor
% slip = - — — - X ioo.

r.p.m. of stator

The greater the load on the pulley wheel, the greater the slip,
and the more rapid is the cutting of the rotor conductors through
the stator flux, and the higher the e. m. f . and consequent current
induced in them.

The increase of current in the rotor which accompanies an
increase of load supply calls for additional power supply. This
is furnished to the stator by the mains automatically, the process
being similar to that by which a load on the secondary of a
transformer (the rotor) increases the current supply to the pri-
mary (the stator). When the rotor is at rest, the induction motor
is essentially a transformer with an unusually large factor of
magnetic leakage. It is the factor of magnetic leakage coupled
with rotor resistance and inductance which prevents the rotor
current from becoming excessive at stand-still. This effect in
turn prevents the stator current from rising to a dangerous value
at stand-still, as it would in a transformer with short-circuited
secondary.

The question of rotor speed and the torque exerted by an
induction motor is a very complicated one, and the various gov-
erning factors must be taken up in detail. First suppose the
torque demanded at the pulley wheel to be doubled. When this
has slowed down the speed to such a degree that the slip has
been doubled, the rotor current, considering resistance alone, is
also doubled, furnishing the required torque.

But other factors enter to disturb this simple ratio between
slip and torque. By the increased slip, the frequency of the
rotor current and consequently the rotor reactance (2w/L) is
increased. This both reduces the current somewhat and changes
its position with reference to the field flux, causing a greater
rotor (armature) reaction than otherwise would be the case and
augmenting the flux leakage. All these factors combine to make
the slip considerably more than twice as great for double torque.

MOTOR AND THE DYNAMO

The rotor reaction in particular is considerable and so distorts
the stator flux that a large part of it passes from pole to pole
through the narrow clearance space between rotor and stator in
such a way as not to act on the rotor inductors.

Thus after a certain point is reached the speed of the induction
motor falls off rapidly with increase of load ; and at a point known
as the pull-out torque, the motor stops altogether.

It will be seen, therefore, that the induction motor is similar
to the direct-current motor in the fact that a decrease of speed
due to load immediately causes an increase of current drawn from
the mains. It is unlike a direct-current motor, on the other

Per cent. slip.

Fig. 147.— Curve of induction motor.

hand, in that when a certain point is reached, the speed falls off
rapidly, and a torque of some 100 per cent, more than that for
which the motor is rated will in most cases bring the machine
to a stand-still.

For any given value of the slip, the torque of an induction motor
varies as the square of the voltage. This comes about from the
fact that the torque is proportional to the product of stator flux
and rotor current. An increase of applied voltage increases this
flux and this in turn increases the rotor current an equal amount,
hence the square. The converse of this proposition is that with

ALTERNATING CURRENT MACHINERY

155

torque constant, the slip will vary inversely with the square of
the applied voltage.

The general shape of the speed torque curve of an induction
motor may be seen in Fig. 147. It will be observed that at about
35 per cent, slip in this case the torque suddenly begins to fall
off, meaning that the so-called pull-out torque has been reached,
and the motor is stopping. A means of shifting this turning
point to correspond to a slower speed is to put resistance in the
rotor circuit. Squirrel-cage rotors having low resistance, are
usually started free and come quickly up to speed, after which
the load may be applied by means of the friction clutch or shifted

'SI

AND <f>

i. ~ PI.

p. 2 MXV.

Fig. 148.

belting. For induction motors intended to start under load, how-
ever, a starting device is required which consists in part at least
of a rotor resistance. The explanation of this principle is as
follows :

As in the transformer, the primary flux of phase I, ^ and the
magnetizing current causing it lag 90° behind the primary
e. m. f. E/i , according to Fig. 148. The secondary e. m. f. E^i
then lags 90° behind $,,. If resistance of the secondary circuit
is zero and inductance alone is present, the secondary current,
!„, lags 90° behind the secondary e. m. f. , and therefore 1 80°

156

THE MOTOR AND THE DYNAMO

behind the primary flux. All this refers to one phase only.
Meanwhile the other phase has been following on 90° behind, so
that the magnetizing current Ip2.mag. and the primary flux <£/2 due
to it are 90° behind 3^ and 90° ahead of Islt Under these cir-
cumstances, the primary flux of phase i can cause no rotation,
being in such a direction as to induce current ISI instead
(Lenz's law), and the primary flux of phase 2 could cause
but little torque, being zero when the rotor current is a
maximum, and vice versa. By increasing the secondary resist-
ance we obtain the phase relation expressed by Fig. 149, and

PI.

s.i

Fig. 149.

although the secondary current is diminished by this means, the
torque is increased.

The rotor resistance, which gives the greatest torque at stand-
still is one such that the resitance and reactance are equal, giving
a power factor angle of 45°. A greater resistance than this
increases the power-factor causing 1^ to swing in closer to Er
in Fig. 149, but at the same time reducing the rotor current so
much that the torque is again decreased.

For the formula of the induction motor and detail calculations
of its construction, the reader is referred to "Electric Motors,"
by Crocker and Arendt, published by Van Nostrandt Co., 1910.

ALTERNATING CURRENT MACHINERY

157

(j) Starters for Polyphase Induction Motors.

For starting polyphase induction motors, the device most
largely used is a resistance in the rotor circuit. In small motors,
up to about 15 horse-power, it may be contained in the space
within the rotor surrounding the shaft. One end of the shaft is
hollow, and a handle protruding through this operates a sliding
contact, which gradually cuts out the resistance, while bringing

Fig. 150.— Forms of rotor with stator for polyphase induction motors. G. E. Co.

the motor up to speed. Resistance coils so placed are never
heavy enough to be more than starting resistances. On larger
machines and in cases where the device is not only for starting,
but is a means of obtaining variable speed, the rotor windings
are brought out to rings with contractors, and wires lead from
these to external resistances. The rotor is usually wound three-
phase, Y connected, and the resistances are varied by means of
a controller. Fig. 150 shows these various types of rotor. Fig.
150 also shows usual startor winding.

158

THE MOTOR AND THE DYNAMO

Fig. 151 shows curves obtained with various resistances in
the rotor circuit. These are so chosen with respect to the rotor
circuit that the total resistance will enable the motor to develop
its maximum torque at standstill (100 per cent, slip), and then
as the speed increases, the resistance may be reduced by the con-
troller in such amounts as to maintain a comparatively constant

0 10 20 30 40 SO 60 70 dO 90 100

Per Cent Synchronism

Fig. 151. — Curves of polyphase induction motor. G. E. Co.

torque. The heavy line represents the portions of the curves used
during this process of starting under load. When the resistance
coils are sufficiently heavy to stand the current without undue
heating, this starting device may be employed to obtain variable
speeds of operating. The usual practice is to build these rheo-
stats for intermittent use from zero to half rated speed of the
motor, and for constant service for half to full speed. Figs.
152 and 153 represent controller and rotor resistance.

ALTERNATING CURRENT MACHINERY 159

Another means of reducing the current at starting is to lessen
the e. m. f . applied to the stator. This is usually accomplished
by means of an auto-transfer in the stator circuit. Figs. 154 and
155 represent one method of connection for a three-phase circuit.

Fig. 152.— Controller for polyphase induction motor. G. E. Co.

When the motor has attained the full speed possible under these
conditions, the stator is thrown directly onto the line by means
of the controller or a double throw switch. This device is
applicable to both squirrel-cage motors and those with wound
rotors, but is clumsy and costly when made for large machines.

i6o

THE; MOTOR AND THE DYNAMO

The Bell Electric Motor Co. of Garwood, N. J., have very
recently placed upon the market a new type of polyphase motor
known as their "Compensated type." The characteristics of this
machine in operating are very interesting. There are two sep-
arate windings on the armature core. One is a progressive wind-
ing somewhat similar to a four pole direct current armature wind-

Fig- 153.— Rotor resistance of polyphase induction motor. G. E. Co.

ing, leads of which are brought out to commutator segments.
Upon this winding but insulated from it there is placed a squirrel-
cage winding of high resistance, which is entirely short-circuited
upon itself. These two windings of high resistance give the motor
considerable torque. After the armature has arrived at a pre-
determined speed, the commutator segments are short-circuited

ALTERNATING CURRENT MACHINERY

161

Generator

Fig. 154.— Starting compensator. Connections for three-wire two-phase. G. E. Co.

- J55- — Starting compensator. Kxterior. G. E. Co.

162

THE MOTOR AND THE DYNAMO

by a centrifugal device. This throws in the entire copper of
the armature, and we then have what are practically two separate
squirrel-cage armatures that are probably in inductive relation
to each other, but are not electrically connected. In starting, all
that is necessary for operation is an ordinary knife-switch, no
compensators, starting boxes or resistances of any kind being
employed. These motors will bring their full-load torque up to
speed on twice full-load current. The power factor and effi-
ciency on all sizes is extremely high. These motors are en-
dorsed by electric-lighting companies, as they do not seriously
interfere with the line voltage, when starting.

(k) The Single-Phase Induction Motor.

Motors of the induction type in sizes up to fifty horse-power
are now manufactured for operating on the single-phase current.
If built in all other respects like a poly-phase motor, such a

TO LINE.

«— REVERSE

TO LI ML

Emerson CatM* JJ3

Fig. 156.— Emerson phase-splitting device. Diagram of connections.

machine would have only an oscillating, not a rotating, field, and
therefore no starting torque, and until a certain speed is attained
it could exert no torque at all. Such a motor, however, if once
brought up to a sufficient speed, will fall into step with its oscil-
lating field, and may then be loaded like any other induction
motor. If the rotor slots are comparatively near together, a
motor of this type may be started, running light, by hand, a few
quick turns by means of the pulley-belt being sufficient. When

ALTERNATING CURRENT MACHINERY 163

the machine has attained full speed, the load is applied by a
sliding belt, or a friction clutch, etc. Other devices, however,
for starting these motors are enumerated below.

The Emerson motor employs a small secondary stator winding

157-— Single-phase stator, showing shading-coils.

of high inductance, in which the current lags about 90° behind
that in the main stator winding, thus producing a rotating field
flux similar to that in the two-phase motor. This is known as
the split-phase method. The secondary winding is cut out auto-
matically, when the motor has attained full speed. See Fig. 156

164

THE: MOTOR AND THE) DYNAMO

For small fan motors, a simple phase-splitting device known
as the shading coil is frequently used. It consists of a copper
band placed about one tip of each pole-piece as shown in Fig.
157. It has the effect of causing the flux from the pole-tip
to be in a different phase from the main part of the flux, and
thus to create a torque sufficient to start the motor.

One of the best starting devices for single-phase induction
motors is that of the General Electric Co. represented in Fig. 158.
The stator is wound as if for three-phase, one of the terminals
being excited through a "condenser-compensator," which has a

To motor.

Fig. 158.— Condenser-compensator starter.

phase-splitting effect, reducing the angle of lag for a portion of
the current. One form of this starting-box requires the handle to
be pressed down for starting. When the motor has attained full
speed, the handle is released, and a spring lifts it clear of the
contact of the compensator circuit.

The Repulsion Motor. — A form of single-phase induction motor
which develops a considerable starting-torque is made with a
rotor in all respects like the armature of a direct-current machine
with a commutator at its end, the same as that of a single-phase
induction motor. The brushes are placed at an angle and per-
manently short-circuited, as represented in Fig. 159. The usual

ALTERNATING CURRENT MACHINERY 165

method of explaining the action of this motor is to consider the
alternating flux, 3>, which is produced by the stator current, as
made up of two components. Of these the component 3>, acts

Fig. 159.— Repulsion motor.

like a transformer flux, inducing current in the short-circuited
armature; and the component ®,n acts like the ordinary field flux
of any motor, exerting a torque on the armature inductors.

One of the chief uses of this type of motor is that it furnishes
a starting device for the single-phase induction motor, when
required to exert a large starting torque. The first manufac-

Fig. 160. — Bell short-circuiting device.

turers to develop this method of starting were the Wagner Elec-
tric Co., but this type of machine is now manufactured by others.
The method consists in applying to the armature of the ordinary
repulsion motor a short-circuiting ring, which is pushed into place

i66

THE MOTOR AND THE DYNAMO

Fig. 161.— 5 h-p. single phase motor.

Test Curve of 5 H. P., 1800 R P M., 220 Volts, 60 Cycles Bell High Efficiency Single Phase Motor

Fig. 162.

ALTERNATING CURRENT MACHINERY

I67

against the commutator bars when the motor has attained nearly
synchronous speed. This is done automatically by a centrifugal
device, attached to the shaft. When the motor is at rest, this
ring is removed by a spring. Figs. 160 and 161 show the method
employed by the Bell Electric Motor Co., and Fig. 162 is a set of
curves representing the performance of one of their machines.

(k) Practical Remarks Regarding Induction Motors.

In specifying an induction motor for any definite work, the

Fig. 163.— 1,400 k.w. motor-generator set with n,ooo-volt induction motor. G. E. Co.

engineer has to consider the two following points : First, the
machine must be large enough to develop the full torque that
will be demanded of it, which can in most cases not exceed more
than 200 per cent, of the rated full load torque of the machine;
and second, the motor must not be larger than actually necessary,
12

1 68

THE MOTOR AND THE DYNAMO

because induction motors act with low power factor and low
efficiency, when the load is much below rated value.

Induction motors made a few years ago were considerably
larger of frame than those now manufactured with the same

Fig. 164.— Motors geared to a lo-foot vertical boring mill. Main motor 15 h-p.
Elevating motor 3 h-p. Westinghouse Co.

power rating. The present practice of manufacturers is to have
stated sizes of rotor and stator punchings of sheet iron, and to
make one size punching serve for two or more sizes of motor,
according to the number of such laminations used. In this type
of machine the open form furnishes good ventilation, the lamina-

ALTERNATING CURRENT MACHINERY 169

tions being bound together by horizontal rods secured to the
end-plates. See Fig. 163.

Fig. 164 shows some applications of the induction motor.

(1) The A. C. Series Motor.

The action of the alternating-current series motor and its
characteristic curves are not far different from those of the
direct-current series motor. Owing to the low flux density of
alternating-current machinery generally, the alternating-current
series motor weighs considerably more and is larger than its
direct-current counterpart.

In the action of the alternating-current series motor, the fol-
lowing peculiarities are to be noted: —

The iron losses are much larger than in the direct-current
machine, owing to the alternating flux in not only the armature
core but also the field. On this account, the field core has to
be laminated, which considerably increases its size.

Besides the c. e. m. f . always present in a rotating armature,
there is developed in the armature windings another e. m. f . by
transformer action from the alternating field-flux. This e. m. f .
neither aids nor opposes the counter e. m. f., the division of arma-
ture inductors in respect to direction of this e. m. f. being at
right angles to the axis of commutation, so that one half counter-
acts the other. This transformer e. m. f., however, greatly
increases the tendency to spark in the coil short-circuited by the
brushes.

The current in the short-circuited coils is sometimes reduced
by inserting a high resistance where the armature coils are con-
nected to the commutator bars.

Another device found in series alternating-current motors is
the compensating winding. It consists of several turns of wire
let into grooves in the pole faces, and serves to reduce armature
reaction and the self-induction of both field and armature circuits.

Figs. 165 and 166 enable the reader to make a comparison of
the characteristics of the alternating-current and the direct-cur-
rent series motor.

170

THE MOTOR AND THE DYNAMO

It is sometimes necessary to operate alternating-current series
motors on a direct-current circuit, as in the case of electric loco-

'•^Q£±

Clou

rw.
C&

f*ffWAT(/Q£

rist/c

•£&£_

OH6>.

O VOi TS

G£M

/AM

TEK

TFWHi.

•£L9.

4-0

o /. \0 / ^ /.

Fig. 165.— Direct-current motor. G. E. Co.

motives operating on two differently equipped roads. Small fan

ALTERNATING CURRENT MACHINERY

motors and the like also capable of operating on either alternating-
current or direct-current circuit are now manufactured.

Fig. 166.— Alternating-current motor. G. E. Co.

INDEX

PAGE

Absolute unit of current 5

Absolute unit of e. m. f. 5

Alternating-current, definition of 105

generation of . 14, 120

measurement of 108-1 13

Alternating e. m. f 107

A. C. circuits in parallel 119

A. C. circuits in series 1 18

ALTERNATORS :

Compound 130-131

Operation in parallel 136

Single-phase 14, 120, 129

Polyphase 120-125

Voltage regulation of 125-131

Angles of lag and lead 50-107

Arc-light dynamos — 64

Armature, construction of 23-33> I24

winding 25-33, 124

Armature characteristic 53

Armature reaction in generators 49

in motors 50

Auto-transformer 150

Balancer 97

Bearings 37

Booster 63

Brush-holders 35-37

Brush generator 65

Brushes 35, 45

number of 31, 32

adjustment of 50, 35

Building-up conditions for 40, 41

Building-up curve 46

Capacity in A. C. circuits 112

Cast iron, magnetic properties 21

CHARACTERISTIC :

Armature 53

External 51

Of alternators 1 25

Of D. C. generators .' 47-55

Of A. C. motors 132, 153, 154, 158, 166, 171

Of D. C. motors 74-78, 83

174 INDEX

PAGE
Circuits, A. C. in parallel 119

A. C. in series 118

Clock diagrams 107

Collector rings 14

Commutation, axis of 50

Commutator, care of 39

construction of 33-35

Compensating winding 131

Compensator for induction motors 161, 164

Compound D. C. generator 54-6i

Componnd motor 76-78

Compounding of alternators 130

of D. C. generators 55

Constant current generator 64

Constant voltage generator 55

Control of generator voltage 53

Control of motor speed 79

Control, series parallel 95

multiple unit 96

Controllers 90-97

Counter-e. m. f. in motors 15, 66

Curves of A. C. current and voltage 108

Curves, (See Characteristics)

Current in A. C. circuits 119, 121, 123

Delta connections 122

Differential motor 77

Drop of voltage in armature 15, 45, 51

Drum-type armature 24

DYNAMO : (See also Generator)

Fundamental equation 14

general construction of D. C. 12-17

general construction of A. C 14, 124

management and operation 39, 53

Eddy-currents 23, 98

Eddy-current losses 98

Edison three-wire system 63

Effective A. C. volts and amperes 108

Efficiency of generators 98

of motors 73-76, 83, 100, 166, 170, 171

by brake test 74

by stray power test 98

Electrical horse-power 74

INDEX 175

PAGE

Electro-motive force, induced 5

of generators !4

in A. C. circuits 109

Electro-magnetism y

Equalizer 6r

Exciter for alternators 1 2 r

External characteristic 52

Farad, definition II2

Faults in armature 42

Faults in connections 40

Field-core 17-20

Field-core losses 9g

Field-magnets 2 r ^y

Field-rheostats 89-91

Field-windings 15, 21

Flat compounding 1-4

Flux and flux density 4

Flux in solenoid g

Foucault currents 2^, 98

Fundamental equation of the generator !4

of the motor I5

General formula of dynamo .-. . ™

GENERATORS :

Alternating-current l^ I20

Direct-current I2

Characteristics of 47-55

Compound 54

Edison three-wire 63

Efficiency of I00

In parallel, A. C. I^

In parallel, D. C. 58-62

Shunt 51-54

Series 64

Troubles of 40

Grounds 42

H and B curves . 9, 47

Harmonic current IOc

Henry, definition of ITQ

Hunting of A. C. machines j^y

Hysteresis 9> Io

Induced electro-motive force 5

Inductors on armature 30

Inductance, definition of I lo

effect of in circuit 1 10

176 INDEX

PAGE

Induction motor 150

polyphase 151, 156

single phase 162

speed control of 158

starting devices 157-164

Inductor alternator 129

Insulation tests 42

Iron losses in dynamos 98

Iron, magnetic characteristics 9

Interpole motor . • • • 83

Lagging current 107

Lap-winding 31

Leading current 112

Lead of brushes 50

Leakage, magnetic 49

Lenz's law 3,7

Lines of force i

Load losses in dynamos and motors - 98

Losses in dynamo machinery 98

Magnetic circuit in dynamos 21

Magnetic field in solenoid 8

Magnetic flux 4

hysteresis 10

permeability 10

properties of materials 21

reluctance 8

Magnetism, residual 41

Magnetization curve 48

Magneto-motive force 8

MOTORS :

Alternating-current 132-136, 150-171

Direct-current 67-97

Characteristics of 75, 76, 154, 166, 171

Compound 78

Induction 150-169

Repulsion 164

Series 72, 76, 77, 171

Shunt 72, 75

Speed control 79-90

Starting-boxes for 90

Synchronous 132

Torque of 70

Motor-generator 97

Non-inductive circuit 1 16

INDEX 177

PAGE

Ohm's law 1 1

Oscillograph 117

Over-compound generator 55

Parallel operation of alternators 136

of D. C. generators 58-62

Permeability 10

Phase in alternating-current 120-123

Poles of dynamo 12, 17

Polyphase alternator 124

Polyphase current 120-123

Power factor • 114

Power in A. C. circuits 114, 116, 117

Power losses in dynamo machines • • • 98

Power of motors 70

Railway motors 93

Railway motor control 95-97

Reactance, definition of no

effect of in

Regulation of generator voltage 53, 126

of motor speeds 71

of voltage in A. C. generators 126, 131

Reluctance, definition of 9

Residual magnetism 41

Resistance, effect of 1 1

measurement of i r

Resistance in rotor of A. C. motors 155

Rheostats for dynamo fields 90

Rheostat control of motors 71, 86, 92, 158

Rotary converter 140

Rotor of induction motor 152, 157

Saturation, magnetic , 47

Self-induction no

Series generators 64

Series motors 72, 93, 169

A. C. and D. C. compared 170, 171

Series motor starters 95

Shunt generators , 51-54

Shunt motors . . . 72, 76, 77, 151

Single phase motors 162-167, ^9

Single phase motor starting devices 163-165

Slip in induction motors 153

Sparking, cause of 56

prevention of, in motors 82

Speed control of motors 79, 90, 96, 158

178 INDEX

PAGE

Split-phase starting device 163

Squirrel-cage rotor 152

Starting-boxes (See Controllers).

Steel, magnetic properties of 21

Steinmetz's formula 10

Step-up and step-down of A. C. e. m. f. 147

Synchronizing , 133

Synchronous converter (See rotary converter).

Synchronous motor 132

Synchronous reactance of alternator 129

Synchroscope 137

Three-phase 122

Tyrrell regulator 131

Traction motors 93-97

Transformers 146-150

Transformer losses 148

Two-phase 120

Variable speed motors 79-90, 158

Voltage in A. C. circuits 118, 122, 123

Voltage control of generators 53, 55, 130

Voltage regulation of generators 126

Voltmeters 115

Wattmeters 117

Watts in A. C. circuits 114-117

Wattless current 1 16

Wave-winding 32

Winding armatures 25-33, I24

Wrought iron, magnetic properties 21

Y-connection 122, 123

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